Force detection device, robot, and moving object

ABSTRACT

A force detection device includes a charge output element that outputs charge in accordance with a received external force, a conversion and output circuit, having a first switching element and a first capacitor, which converts the charge into a voltage and outputs the voltage, a compensation signal output circuit, having a second switching element and a second capacitor, which outputs a compensation signal, and an external force detection circuit that detects an external force on the basis of the voltage which is output from the conversion and output circuit and the compensation signal which is output from the compensation signal output circuit. The capacitance of the second capacitor is smaller than the capacitance of the first capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/181,915, filed Feb. 17, 2014, which claims priority to JapanesePatent Application No. 2013-029728, filed Feb. 19, 2013, and JapanesePatent Application No. 2013-036773, filed Feb. 27, 2013, all of whichare hereby expressly incorporated by reference herein in theirentireties.

BACKGROUND

1. Technical Field

The present invention relates to a force detection device, a robot, anda moving object.

2. Related Art

In recent years, for the purpose of an improvement in productionefficiency, the introduction of industrial robots to productionfacilities such as a factory has progressed. Such industrial robotsinclude an arm capable of being driven in one-axis or plural-axisdirections, and an end effector, such as a hand, a component inspectioninstrument or a component transport instrument, which is installed atthe tip side of the arm, and can execute component assembly work,component manufacturing work such as component machining work, componenttransport work, component inspection work, and the like.

In such industrial robots, a force detection device is provided betweenthe arm and the end effector. As a force detection device used in theindustrial robots, for example, a force detection device disclosed inJP-A-5-95237 is used. The force detection device of JP-A-5-95237 isconstituted by a charge output element that outputs charge in accordancewith a received external force, an amplifier that amplifies the chargewhich is output from the charge output element, a capacitor forconverting the charge which is output from the charge output elementinto a voltage, and a reset circuit having a mechanical relay forshort-circuiting terminals of the capacitor and resetting chargeaccumulated in the capacitor. With such a configuration, the forcedetection device of JP-A-5-95237 can detect an external force appliedalong any one axis to the charge output element.

However, in order to control the end effector of the industrial robot,it may be necessary to detect six-axis forces (translational forcecomponents in the directions of x, y, and z axes and rotational forcecomponents around the x, y, and z axes). In such a case, it is necessaryto form a three-axis force detection device capable of detectingthree-axis forces (translational forces in the directions of the x, y,and z axes) by combining at least three force detection devices asdisclosed in JP-A-5-95237, and to mount at least three three-axis forcedetection devices to the wrist of the industrial robot.

When the size of the force detection device mounted to the wrist of suchan industrial robot is large, the operating area of the wrist may becomenarrower. In addition, when the size of the force detection device islarge, the distance from the joint of the industrial robot to the end ofthe end effector becomes longer, and thus the load capacity of theindustrial robot may be reduced. Therefore, it is preferable that theforce detection device is small in size and light in weight.

In order to solve such problems, various methods are proposed. Forexample, JP-A-11-148878 discloses a force detection device using asemiconductor switching element as a reset circuit. Since thesemiconductor switching element is smaller in size and lighter in weightthan the mechanical relay, it is possible to reduce the size and weightof the entire device by using the semiconductor switching element as thereset circuit.

However, when the semiconductor switching element is used as the resetcircuit, an output drift due to a leakage current of the semiconductorswitching element is generated. Such an output drift deteriorates thedetection resolution and detection accuracy of the force detectiondevice, which leads to undesirable results. Further, since the outputdrift is accumulated in proportion to the measurement (operating) timeof the force detection device, there is a problem in that the measurabletime of the force detection device cannot be lengthened.

In addition, as the force detection device, a quartz crystalpiezoelectric sensor using quartz crystal as a charge output element iswidely used. The quartz crystal piezoelectric sensor has characteristicsexcellent in a wide dynamic range, high rigidity, high naturalfrequency, and high load bearing capacity, and thus is widely used inthe industrial robot.

However, in such a quartz crystal piezoelectric sensor, charge which isoutput from quartz crystal is weak, and thus it is not possible toignore the influence of an output drift caused by the leakage current ofa conversion and output circuit. Various methods for reducing the outputdrift have been examined. For example, JP-A-9-72757 discloses a quartzcrystal piezoelectric sensor provided with a reverse bias circuit usinga diode having current characteristics similar to the characteristics ofthe leakage current of the conversion and output circuit. In the quartzcrystal piezoelectric sensor disclosed in JP-A-9-72757, a correctioncurrent which has a size substantially equal to that of the leakagecurrent of the conversion and output circuit and of which the flowdirection is opposite thereto is supplied from the diode, and thus anoutput drift is reduced.

However, when the reverse bias circuit is used as in the quartz crystalpiezoelectric sensor disclosed in JP-A-9-72757, additional componentssuch as the diode are required, and the mounting area thereof isexpanded, which leads to the difficulty of a reduction in size. Inaddition, there is a problem in that component quality control forsupplying a desired correction current is required.

SUMMARY

An advantage of some aspects of the invention is to provide a forcedetection device having a small size and a reduced output drift, a robotusing the force detection device, and a moving object.

An aspect of the invention is directed to a force detection deviceincluding: a charge output element that outputs charge in accordancewith an external force; a conversion and output circuit, having a firstcapacitor, which converts the charge into a voltage and outputs thevoltage; a compensation signal output circuit, having a secondcapacitor, which outputs a compensation signal; and an external forcedetection circuit that detects the external force on the basis of thevoltage which is output from the conversion and output circuit and thecompensation signal which is output from the compensation signal outputcircuit. A capacitance of the second capacitor is smaller than acapacitance of the first capacitor.

With this configuration, it is possible to compensate for the voltagewhich is output from the conversion and output circuit, using thecompensation signal which is output by the compensation signal outputcircuit. As a result, it is possible to perform higher-accuracy forcedetection. In addition, since the capacitance of the second capacitor issmaller than the capacitance of the first capacitor, the compensationsignal output circuit can more accurately acquire the compensationsignal from the second switching element. As a result, it is possible tomore accurately compensate for the voltage which is output from theconversion and output circuit.

In the force detection device, it is preferable that when thecapacitance of the first capacitor is set to C1, and the capacitance ofthe second capacitor is set to C2, C2/C1 is 0.1 to 0.8.

When the capacitance ratio C2/C1 falls below the lower limit, the secondcapacitor may be saturated. On the other hand, when the capacitanceratio C2/C1 exceeds the upper limit, the compensation signal may not beable to be accurately acquired from the second switching element.

In the force detection device, it is preferable that the external forcedetection circuit includes a gain correction portion that gives a gainto at least one of the voltage which is output from the conversion andoutput circuit and the compensation signal which is output from thecompensation signal output circuit, to perform correction, and theexternal force detection circuit detects the external force on the basisof the voltage corrected by the gain correction portion and thecompensation signal.

With this configuration, it is possible to correct a sensitivitydifference between the voltage and the compensation signal which iscaused by the difference between the capacitance C1 of the firstcapacitor and the capacitance C2 of the second capacitor.

Another aspect of the invention is directed to a force detection deviceincluding: a first element and a second element that output voltages inaccordance with an external force; and an external force detectioncircuit that detects the external force on the basis of the voltageswhich are output from the first element and the second element. Thefirst element and the second element include a piezoelectric substance,having an electric axis, which outputs charge in accordance with theexternal force along the electric axis, and a conversion and outputcircuit that converts the charge which is output from the piezoelectricsubstance into the voltage. The first element and the second element aredisposed so that a direction of the electric axis included in thepiezoelectric substance of the first element and a direction of theelectric axis included in the piezoelectric substance of the secondelement are opposite to each other.

With this configuration, the sign of the output drift included in thevoltage which is output from the first element and the sign of theoutput drift included in the voltage which is output from the secondelement are consistent with each other, but it is possible to reversethe sign of a voltage component (true value), included in the voltagewhich is output from the first element, which is proportional to theaccumulated amount of the charge which is output from the piezoelectricsubstance in accordance with the external force and the sign of avoltage component (true value), included in the voltage which is outputfrom the second element, which is proportional to the accumulated amountof the voltage which is output from the piezoelectric substance inaccordance with the external force. Therefore, the external force iscalculated using the voltage which is output from the first element andthe voltage which is output from the second element, and thus it ispossible to detect the external force while reducing the output driftincluded in the voltages which are output from the first element andsecond element. As a result, it is possible to improve the detectionaccuracy and detection resolution of the force detection device.Further, since a circuit, such as a reverse bias circuit, for reducingthe output drift is not required, it is possible to reduce the size ofthe force detection device.

In the force detection device, it is preferable that the first elementand the second element are disposed so that the direction of theelectric axis of the piezoelectric substance of the first element andthe direction of the electric axis of the piezoelectric substance of thesecond element face each other on the same axis.

With this configuration, it is possible to detect the external forcewhile further reducing the output drift.

In the force detection device, it is preferable that each of thepiezoelectric substances includes: a first piezoelectric plate which hasa first crystal axis; a second piezoelectric plate, provided facing thefirst piezoelectric plate, which has a second crystal axis; and aninternal electrode provided between the first piezoelectric plate andthe second piezoelectric plate, and the first crystal axis of the firstpiezoelectric plate has a polarity different from that of the secondcrystal axis of the second piezoelectric plate.

With this configuration, it is possible to increase positive charge ornegative charge collected in the vicinity of the internal electrode.

In the force detection device, it is preferable that the external forcedetection portion detects the external force applied to the forcedetection device by taking a difference between the voltages convertedfrom the charge which is output from the first element and the secondelement.

With this configuration, it is possible to reduce a detection errorcaused by the output drift.

In the force detection device, it is preferable that when a laminationdirection of the piezoelectric substance is set to a γ-axis direction,and directions which are orthogonal to the γ-axis direction and areorthogonal to each other are set to an α-axis direction and a β-axisdirection, respectively, one of the piezoelectric substances is anα-axis piezoelectric substance that outputs the charge in accordancewith the external force along the α-axis direction, one of thepiezoelectric substances is a β-axis piezoelectric substance thatoutputs the charge in accordance with the external force along theβ-axis direction, and one of the piezoelectric substances is a γ-axispiezoelectric substance that outputs the charge in accordance with theexternal force along the γ-axis direction.

With this configuration, the piezoelectric substances can output thecharge in accordance with three-axis forces (translational forcecomponents in the directions of the x, y, and z axes).

In the force detection device, it is preferable that the force detectiondevice includes two first elements and two second elements, thedirection of the electric axis of the α-axis piezoelectric substance ofone of the first elements and one of the second elements is opposite tothe direction of the electric axis of the α-axis piezoelectric substanceof the other of the first elements and the other of the second elements,and the direction of the electric axis of the γ-axis piezoelectricsubstance of the one of the first elements and the one of the secondelements is opposite to the direction of the electric axis of the γ-axispiezoelectric substance of the other of the first elements and the otherof the second elements.

With this configuration, it is possible to detect six-axis forces whilereducing the output drift on the basis of the voltages which are outputfrom the first element and the second element.

In the force detection device, it is preferable that the force detectiondevice includes a base plate and a cover plate provided separately fromthe base plate, to which the external force is given, and each of theelements is provided between the base plate and the cover plate.

With this configuration, it is possible to detect the external forceapplied to the base plate or the cover plate.

In the force detection device, it is preferable that each of theelements is disposed at equal angular intervals along a circumferentialdirection of the base plate or the cover plate.

With this configuration, it is possible to detect the external force inan unbiased manner.

Still another aspect of the invention is directed to a robot including:at least one arm connecting body having a plurality of arms andconfigured to rotatably connect adjacent arms of the plurality of arms;an end effector provided at a tip side of the arm connecting body; andthe force detection device of any of the configurations described above,which is provided between the arm connecting body and the end effectorand detects an external force applied to the end effector.

In the robot, the external force detected by the force detection deviceis fed back, and thus it is possible to more precisely execute work. Inaddition, it is possible to detect the contact of the end effector to anobstacle, and the like, through the external force detected by the forcedetection device. Therefore, it is possible to easily perform anobstacle avoidance operation, an object damage avoidance operation andthe like which are difficult to perform in the position control of therelated art, and to execute work more safely.

Yet another aspect of the invention is directed to a moving objectincluding: a power output portion that supplies power for movement; andthe force detection device of any of the configurations described above,which detects an external force generated by the movement.

In the moving object, it is possible to detect an external force causedby vibration, acceleration and the like which are generated with themovement, and the moving object can execute control such as posturecontrol, vibration control and acceleration control. Further, since acircuit, such as a reverse bias circuit, for reducing the output driftis not required, it is possible to reduce the size of the forcedetection device. Therefore, it is possible to reduce the size of themoving object.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a circuit diagram schematically illustrating a firstembodiment of a force detection device according to the invention.

FIG. 2 is a cross-sectional view schematically illustrating a chargeoutput element of the force detection device shown in FIG. 1.

FIGS. 3A and 3B are a plan view and a cross-sectional view,respectively, illustrating a mounting example of capacitors of the forcedetection device shown in FIG. 1.

FIG. 4 is a circuit diagram schematically illustrating a secondembodiment of the force detection device according to the invention.

FIG. 5 is a cross-sectional view schematically illustrating a chargeoutput element of the force detection device shown in FIG. 4.

FIG. 6 is a perspective view schematically illustrating a thirdembodiment of the force detection device according to the invention.

FIGS. 7A and 7B are a perspective view and a plan view, respectively,schematically illustrating a fourth embodiment of the force detectiondevice according to the invention.

FIG. 8 is a circuit diagram schematically illustrating the forcedetection device shown in FIGS. 7A and 7B.

FIG. 9 is a cross-sectional view schematically illustrating a chargeoutput element of the force detection device shown in FIGS. 7A and 7B.

FIGS. 10A and 10B are a perspective view and a plan view, respectively,schematically illustrating a fifth embodiment of the force detectiondevice according to the invention.

FIG. 11 is a circuit diagram schematically illustrating the forcedetection device shown in FIGS. 10A and 10B.

FIGS. 12A and 12B are cross-sectional views schematically illustratingcharge output elements of the force detection device shown in FIGS. 10Aand 10B.

FIG. 13 is a diagram illustrating an example of a single-arm robot usingthe force detection device according to the invention.

FIG. 14 is a diagram illustrating an example of a moving object usingthe force detection device according to the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a force detection device according to the invention will bedescribed in detail on the basis of preferred embodiments shown in theaccompanying drawings.

First Embodiment

FIG. 1 is a circuit diagram schematically illustrating a firstembodiment of the force detection device according to the invention.FIG. 2 is a cross-sectional view schematically illustrating a chargeoutput element of the force detection device shown in FIG. 1. FIGS. 3Aand 3B are diagrams illustrating a mounting example of capacitors of theforce detection device shown in FIG. 1; FIG. 1A is a plan view, and FIG.1B is a cross-sectional view.

A force detection device 1 a shown in FIG. 1 has a function of detectingan external force applied along any one axis (x-axis, y-axis or z-axis).The force detection device 1 a includes a charge output element 10 athat outputs charge Q in accordance with an external force applied(received) along any one axis, a conversion and output circuit 20 thatconverts the charge Q which is output from the charge output element 10a into a voltage V and outputs the voltage V, a compensation signaloutput circuit 30 that outputs a compensation signal Voff, and anexternal force detection circuit 40 a that detects the applied externalforce on the basis of the voltage V which is output from the conversionand output circuit 20 and the compensation signal Voff which is outputfrom the compensation signal output circuit 30.

Charge Output Element

The charge output element 10 a shown in FIG. 2 has a function ofoutputting the charge Q in accordance with an external force (shearingforce) applied (received) along a β-axis in FIG. 2. The charge outputelement 10 a includes two ground electrode layers 11 and a piezoelectricsubstance 12 provided between the two ground electrode layers 11.Meanwhile, in FIG. 2, the lamination direction of the ground electrodelayers 11 and the piezoelectric substance 12 is set to a γ-axisdirection, and the directions which are orthogonal to the γ-axisdirection and are orthogonal to each other are set to an α-axisdirection and a β-axis direction, respectively.

In the shown configuration, both the ground electrode layer 11 and thepiezoelectric substance 12 have the same width (length in a horizontaldirection in the drawing), but the invention is not limited thereto. Forexample, the width of the ground electrode layer 11 may be greater thanthe width of the piezoelectric substance 12, or vice versa.

The ground electrode layer 11 is an electrode grounded to a ground(reference potential point) GND. Materials constituting the groundelectrode layer 11, though not particularly limited, are preferably, forexample, gold, titanium, aluminum, copper, iron or an alloy containingthese materials. Among these materials, particularly, it is preferableto use stainless steel which is an iron alloy. The ground electrodelayer 11 formed of stainless steel has excellent durability andcorrosion resistance.

The piezoelectric substance 12 has a function of outputting the charge Qin accordance with the external force (shearing force) applied(received) along the β-axis. The piezoelectric substance 12 isconfigured to output positive charge in accordance with an externalforce applied along the positive direction of the β-axis, and to outputnegative charge in accordance with an external force applied along thenegative direction of the β-axis.

The piezoelectric substance 12 includes a first piezoelectric plate 121having a first crystal axis CA1, a second piezoelectric plate 123,provided facing the first piezoelectric plate 121, which has a secondcrystal axis CA2, and an internal electrode 122, provided between thefirst piezoelectric plate 121 and the second piezoelectric plate 123,which outputs the charge Q.

The first piezoelectric plate 121 is constituted by a piezoelectricsubstance having the first crystal axis CA1 oriented in the negativedirection of the β-axis. When the external force along the positivedirection of the β-axis is applied to the surface of the firstpiezoelectric plate 121, charge is induced into the first piezoelectricplate 121 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the first piezoelectricplate 121 on the internal electrode 122 side, and negative charge iscollected in the vicinity of the surface of the first piezoelectricplate 121 on the ground electrode layer 11 side. Similarly, when theexternal force along the negative direction of the β-axis is applied tothe surface of the first piezoelectric plate 121, negative charge iscollected in the vicinity of the surface of the first piezoelectricplate 121 on the internal electrode 122 side, and positive charge iscollected in the vicinity of the surface of the first piezoelectricplate 121 on the ground electrode layer 11 side.

The second piezoelectric plate 123 is constituted by a piezoelectricsubstance having the second crystal axis CA2 oriented in the positivedirection of the β-axis. When the external force along the positivedirection of the β-axis is applied to the surface of the secondpiezoelectric plate 123, charge is induced into the second piezoelectricplate 123 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the second piezoelectricplate 123 on the internal electrode 122 side, and negative charge iscollected in the vicinity of the surface of the second piezoelectricplate 123 on the ground electrode layer 11 side. Similarly, when theexternal force along the negative direction of the β-axis is applied tothe surface of the second piezoelectric plate 123, negative charge iscollected in the vicinity of the surface of the second piezoelectricplate 123 on the internal electrode 122 side, and positive charge iscollected in the vicinity of the surface of the second piezoelectricplate 123 on the ground electrode layer 11 side.

In this manner, the direction of the first crystal axis CA1 of the firstpiezoelectric plate 121 is opposite to the direction of the secondcrystal axis CA2 of the second piezoelectric plate 123. Thereby, it ispossible to increase the positive charge or the negative chargecollected in the vicinity of the internal electrode 122, as comparedwith a case where the piezoelectric substance 12 is constituted by onlyany one of the first piezoelectric plate 121 and the secondpiezoelectric plate 123, and the internal electrode 122. As a result, itis possible to increase the charge Q which is output from the internalelectrode 122.

Meanwhile, constituent materials of the first piezoelectric plate 121and the second piezoelectric plate 123 include quartz crystal, topaz,barium titanate, lead titanate, lead zirconate titanate (PZT:Pb(Zr,Ti)O₃), lithium niobate, lithium tantalite, and the like. Among thesematerials, particularly, quartz crystal is preferable. This is because apiezoelectric plate formed of quartz crystal has characteristicsexcellent in a wide dynamic range, high rigidity, high naturalfrequency, and high load bearing capacity. In addition, a piezoelectricplate, such as the first piezoelectric plate 121 and the secondpiezoelectric plate 123, which generates charge by an external force(shearing force) applied along the surface direction of the layer can beformed of Y cut quartz crystal.

The internal electrode 122 has a function of outputting positive chargeor negative charge, generated within the first piezoelectric plate 121and the second piezoelectric plate 123, as the charge Q. As describedabove, when the external force along the positive direction of theβ-axis is applied to the surface of the first piezoelectric plate 121 orthe surface of the second piezoelectric plate 123, positive charge iscollected in the vicinity of the internal electrode 122. As a result,positive charge Q is output from the internal electrode 122. On theother hand, when the external force along the negative direction of theβ-axis is applied to the surface of the first piezoelectric plate 121 orthe surface of the second piezoelectric plate 123, negative charge iscollected in the vicinity of the internal electrode 122. As a result,negative charge Q is output from the internal electrode 122.

In addition, the width of the internal electrode 122 is preferably equalto or greater than the widths of the first piezoelectric plate 121 andthe second piezoelectric plate 123. When the width of the internalelectrode 122 is smaller than that of the first piezoelectric plate 121or the second piezoelectric plate 123, a portion of the firstpiezoelectric plate 121 or the second piezoelectric plate 123 is not incontact with the internal electrode 122. For this reason, a portion ofcharge generated in the first piezoelectric plate 121 or the secondpiezoelectric plate 123 may not be able to be output from the internalelectrode 122. As a result, the charge Q which is output from theinternal electrode 122 decreases.

In this manner, the charge output element 10 a includes the groundelectrode layer 11 and the piezoelectric substance 12 mentioned above,and thus can output the charge Q in accordance with the external forceparallel or substantially parallel to the β-axis in FIG. 2.

Meanwhile, an example has been described in which the charge outputelement 10 a has a function of outputting the charge Q in accordancewith the external force (shearing force) parallel or substantiallyparallel to the β-axis, but the invention is not limited thereto. Byusing the first piezoelectric plate 121 in which the orientationdirection of the first crystal axis CA1 is different from the β-axisdirection and the second piezoelectric plate 123 in which theorientation direction of the second crystal axis CA2 is different fromthe β-axis direction, it is possible to form the charge output element10 a that outputs the charge Q in accordance with an external force(shearing force) parallel or substantially parallel to an α-axis or anexternal force (compressive/tensile force) parallel or substantiallyparallel to a γ-axis. Such a case is also within the scope of theinvention.

Conversion and Output Circuit

The conversion and output circuit 20 has a function of converting thecharge Q which is output from the charge output element 10 a into avoltage V and outputting the voltage V. The conversion and outputcircuit 20 includes an operational amplifier 21, a capacitor 22 as afirst capacitor, and a switching element 23. A first input terminal(negative input) of the operational amplifier 21 is connected to theinternal electrode 122 of the charge output element 10 a, and a secondinput terminal (positive input) of the operational amplifier 21 isgrounded to a ground (reference potential point). In addition, an outputterminal of the operational amplifier 21 is connected to the externalforce detection circuit 40 a. The capacitor 22 is connected between thefirst input terminal and the output terminal of the operationalamplifier 21. The switching element 23 is connected between the firstinput terminal and the output terminal of the operational amplifier 21,and is connected in parallel with the capacitor 22. In addition, theswitching element 23 is connected to a drive circuit (not shown), andthe switching element 23 executes a switching operation in accordancewith an on/off signal from the drive circuit.

When the switching element 23 is turned off, the charge Q which isoutput from the charge output element 10 a is accumulated in thecapacitor 22 having a capacitance C1, and is output to the externalforce detection circuit 40 a as the voltage V. Next, when the switchingelement 23 is turned on, both terminals of the capacitor 22 areshort-circuited therebetween. As a result, the charge Q accumulated inthe capacitor 22 is discharged to be held at 0 coulombs, and the voltageV which is output to the external force detection circuit 40 a is heldat 0 volts. The turn-on of the switching element 23 refers to theresetting of the conversion and output circuit 20.

The switching element 23 is a semiconductor switching element such as aMOSFET (Metal Oxide Semiconductor Field Effect Transistor). Asemiconductor switching element is smaller in size and lighter in weightthan a mechanical switch, and thus has an advantage in reducing the sizeand weight of the force detection device 1 a. Hereinafter, as arepresentative example, a case where a MOSFET is used as the switchingelement 23 will be described.

The switching element 23 has a drain electrode, a source electrode, anda gate electrode. One of the drain electrode and the source electrode ofthe switching element 23 is connected to the first input terminal of theoperational amplifier 21, and the other of the drain electrode and thesource electrode is connected to the output terminal of the operationalamplifier 21. In addition, a gate electrode of the switching element 23is connected to a drive circuit (not shown).

The voltage V which is output from the ideal conversion and outputcircuit 20 is proportional to the accumulated amount of the charge Qwhich is output from the charge output element 10 a. However, in theactual conversion and output circuit 20, a leakage current flowing fromthe switching element 23 into the capacitor 22 is generated. Such aleakage current acts as an output drift D included in the voltage V.Therefore, when a voltage component (true value) proportional to theaccumulated amount of the charge Q is set to Vt, the output voltage Vsatisfies the relation of V=Vt+D.

Since the output drift D is equivalent to an error on a measurementresult, there is a problem in that the detection accuracy and detectionresolution of the force detection device 1 a deteriorate. In addition,since the leakage current is accumulated in proportion to themeasurement (drive) time, there is a problem in that the measurementtime of the force detection device 1 a cannot be lengthened.

Such a leakage current is caused by semiconductor structures such as thelack of insulation properties of a gate insulating film, the refinementof a process rule, and the variation of impurity concentration in asemiconductor, and use environments such as temperature and humidity.The leakage current caused by the semiconductor structure serves as aneigenvalue for each switching element, and thus can be compensated forrelatively easily by measuring the leakage current caused by thesemiconductor structure in advance. However, since the leakage currentcaused by the use environment fluctuates depending on the useenvironment (conditions), it is not likely that the leakage current willbe compensated for. The force detection device 1 a of the embodiment canreduce (compensate for) the influences of the leakage current caused bythe semiconductor structure and the leakage current caused by the useenvironment, using the compensation signal Voff which is output from thecompensation signal output circuit 30 as described next.

Compensation Signal Output Circuit

The compensation signal output circuit 30 has a function of outputtingthe compensation signal Voff for compensating for the voltage V which isoutput from the conversion and output circuit 20. As shown in thedrawing, the compensation signal output circuit 30 may be providedindependently of the conversion and output circuit 20. The phrase“provided independently” as used herein refers to the fact thatcomponents (operational amplifier 31, capacitor 32 as a second capacitorand switching element 33 which are described later) of the compensationsignal output circuit 30 and components (that is, operational amplifier21, capacitor 22 and switching element 23) of the conversion and outputcircuit 20 are elements (components) different from each other. That is,the compensation signal output circuit 30 is provided separately fromthe conversion and output circuit 20, and does not share the componentsthereof with the conversion and output circuit.

The compensation signal output circuit 30 includes the operationalamplifier 31, the capacitor 32 as a second capacitor, and the switchingelement 33. A first input terminal (negative input) of the operationalamplifier 31 is connected to the capacitor 32 and the switching element33, and an input terminal (positive input) of the operational amplifier31 is grounded to a ground (reference potential point). In addition, anoutput terminal of the operational amplifier 31 is connected to theexternal force detection circuit 40 a. The capacitor 32 is connectedbetween the input terminal and the output terminal of the operationalamplifier 31. The switching element 33 is connected between the firstinput terminal and the output terminal of the operational amplifier 31,and is connected in parallel with the capacitor 32. In addition, theswitching element 33 is connected to a drive circuit (not shown), andthe switching element 33 executes a switching operation in accordancewith an on/off signal from the drive circuit.

The switching element 33 is the same semiconductor switching element(MOSFET) as the switching element 23 of the conversion and outputcircuit 20. The switching element 33 has a drain electrode, a sourceelectrode, and a gate electrode. One of the drain electrode and thesource electrode of the switching element 33 is connected to the firstinput terminal of the operational amplifier 31, and the other of thedrain electrode and the source electrode is connected to the outputterminal of the operational amplifier 31. In addition, the gateelectrode of the switching element 33 is connected to a drive circuit(not shown).

The drive circuit connected to the switching element 33 may be the samedrive circuit as the drive circuit connected to the switching element 23of the conversion and output circuit 20, and may be a different drivecircuit. When the drive circuit connected to the switching element 33and the drive circuit connected to the switching element 23 are drivecircuits different from each other, the drive circuit connected to theswitching element 33 outputs an on/off signal synchronous with the drivecircuit connected to the switching element 23. Thereby, the switchingoperation of the switching element 33 and the switching operation of theswitching element 23 are synchronized with each other. That is, theon/off timings of the switching element 33 and the switching element 23are consistent with each other.

The switching element 33 is the same semiconductor switching element asthe switching element 23 of the conversion and output circuit 20.Therefore, the leakage current caused by the semiconductor structure ofthe switching element 33 is substantially equal to the leakage currentcaused by the semiconductor structure of the switching element 23. Thephrase “substantially equal” as used herein means that the differencebetween the leakage current caused by the semiconductor structure of theswitching element 33 and the leakage current caused by the semiconductorstructure of the switching element 23 is sufficiently small to theextent of being negligible as compared with the leakage currents causedby the semiconductor structures of the switching elements 23 and 33.

In addition, the switching element 33 is mounted under the same useenvironment as that of the switching element 23 of the conversion andoutput circuit 20. The phrase “use environment” as used herein refers totemperature and humidity. Thereby, it is possible to make the leakagecurrent caused by the use environment of the switching element 33 andthe leakage current caused by the use environment of the switchingelement substantially equal to each other. The phrase “substantiallyequal” as used herein means that the difference between the leakagecurrent caused by the use environment of the switching element 33 andthe leakage current caused by the use environment of the switchingelement 23 is sufficiently small to the extent of being negligible ascompared with the leakage currents caused by the use environments of theswitching elements 23 and 33.

As a result, the leakage current of the switching element 33 operatessimultaneously with the leakage current of the switching element 23.That is, when the leakage current of the switching element 23 increases,the leakage current of the switching element 33 increases similarly.When the leakage current of the switching element 23 decreases, theleakage current of the switching element 33 decreases similarly.Thereby, the compensation signal output circuit 30 detects the leakagecurrent of the switching element 33, thereby allowing the leakagecurrent of the switching element 23 to be indirectly acquired.

The phrase “under the same use environment” as mentioned above includes,for example, a case where the switching element 33 is mounted in thevicinity of the switching element 23, a case where the switching element23 and the switching element 33 are mounted in the same housing, a casewhere the switching element 23 and the switching element 33 are mountedon the same semiconductor substrate, and the like.

Among these cases, it is preferable that the switching element 23 andthe switching element 33 are mounted on the same semiconductorsubstrate. The switching element 23 and the switching element 33 aremounted on the same semiconductor substrate, thereby allowing thetemperature and humidity around the switching element 23 and thetemperature and humidity around the switching element 33 to be easilymade substantially equal to each other. The phrase “substantially equal”as used herein means that the difference between the temperature andhumidity around the switching element 33 and the temperature andhumidity of the switching element 23 is sufficiently small to the extentof being negligible.

In addition, when the switching element 23 and the switching element 33are mounted on the same semiconductor substrate, the switching element23 and the switching element 33 can be formed in the same process, whichleads to the advantage of shortening a working process. In addition,since the switching element 23 and the switching element 33 can beformed in the same process, it is possible to suppress a variation inthe characteristics of the switching element 23 and the switchingelement 33. Therefore, the leakage current caused by the semiconductorstructure of the switching element and the leakage current caused by thesemiconductor structure of the switching element 33 can be made equal toeach other with a higher degree of accuracy.

When the switching element 33 is turned off, the leakage currentgenerated in the switching element 33 flows into the capacitor 32 havinga capacitance C2. Thereby, charge is accumulated, and thus is output tothe external force detection circuit 40 a as the compensation signalVoff. Next, when the switching element 33 is turned on, both terminalsof the capacitor 32 are short-circuited therebetween. As a result, thecharge Q accumulated in the capacitor 32 is discharged to be held at 0coulombs, and the compensation signal Voff which is output to theexternal force detection circuit 40 a is held at 0 volts.

When the capacitance of a capacitor is reduced in the circuit, such asthe conversion and output circuit 20 or the compensation signal outputcircuit 30, which has a voltage conversion function, voltage conversionsensitivity is improved, but the amount of saturated charge is reduced.Generally, the leakage currents of the semiconductor switching elementssuch as the switching elements 23 and 33 are smaller than the charge Qwhich is input from the charge output element 10 a. Therefore, it ispreferable that the capacitance C2 of the capacitor 32 is smaller thanthe capacitance C1 of the capacitor 22. Thereby, the leakage currentgenerated in the switching element 33 can be converted into a voltagemore accurately.

In addition, the capacitance ratio C2/C1 of the capacitance C2 of thecapacitor 32 to the capacitance C1 of the capacitor 22 is preferably 0.1to 0.8, and is more preferably 0.3 to 0.6. When the capacitance ratioC2/C1 falls below the lower limit, the capacitor 32 may be saturated bythe leakage current generated in the switching element 33. On the otherhand, when the capacitance ratio C2/C1 exceeds the upper limit,sufficient sensitivity to the leakage current generated in the switchingelement 33 may not be obtained.

FIGS. 3A and 3B illustrate an example of a circuit in which twocapacitors 22 and 32 having different capacitances are mounted on thesame semiconductor substrate. FIG. 3A is a plan view of a circuit havingthe capacitor 22 and the capacitor 32. Meanwhile, in FIG. 3A, some ofthe components are shown in perspective for the purpose of description.FIG. 3B is a cross-sectional view taken along line A-A of FIG. 3A. Thecircuit of FIGS. 3A and 3B includes a semiconductor substrate 50,insulating interlayers 60 and 70 provided on the semiconductor substrate50, the capacitors 22 and 32 provided on the insulating interlayer 60,power distribution layers 80 a and 80 b, and through holes 71 providedwithin the insulating interlayer 70.

The capacitor 22 is electrically connected to the operational amplifier21 and the switching element 23 which are not shown in FIG. 3A or 3B,through the power distribution layer 80 a and the through holes 71.Similarly, the capacitor 32 is electrically connected to the operationalamplifier 31 and the switching element 33 which are not shown in FIG. 3Aor 3B, through the power distribution layer 80 b and the through holes71.

The capacitor 22 having the capacitance C1 includes a capacitor lowerelectrode layer 221, two capacitor upper electrode layers 223 facing thecapacitor lower electrode layer 221, and a capacitor insulating layer222 provided between the capacitor lower electrode layer 221 and thecapacitor upper electrode layers 223.

The capacitor 32 having the capacitance C2 includes a capacitor lowerelectrode layer 421, a capacitor upper electrode layer 423 facing thecapacitor lower electrode layer 421, and a capacitor insulating layer422 provided between the capacitor lower electrode layer 421 and thecapacitor upper electrode layer 423.

The capacitance of a capacitor having structures such as the capacitor22 and the capacitor 32 is proportional to the area of the capacitorupper electrode layer. In the shown configuration, the area of thecapacitor upper electrode layer 423 of the capacitor 32 is smaller thanthe area of the capacitor upper electrode layer 223 of the capacitor 22.With such a configuration, two capacitors 22 and 32 having differentcapacitances can be mounted on the same semiconductor substrate 50.

The mounting example of two capacitors 22 and 32 having differentcapacitances has been described with reference to FIGS. 3A and 3B, butthe invention is not limited thereto. For example, the compensationsignal output circuit 30 may have a plurality of capacitors connected inseries with each other. Thereby, it is possible to reduce thecapacitance of the capacitor of the compensation signal output circuit30, and to make the capacitance of the capacitor of the compensationsignal output circuit 30 smaller than the capacitance of the capacitorof the conversion and output circuit 20. In addition, the conversion andoutput circuit 20 may have a plurality of capacitors connected inparallel with each other. Thereby, it is possible to increase thecapacitance of the capacitor of the conversion and output circuit 20,and to make the capacitance of the capacitor of the compensation signaloutput circuit 30 smaller than the capacitance of the capacitor of theconversion and output circuit 20. Such a case is also within the scopeof the invention.

External Force Detection Circuit

The external force detection circuit 40 a has a function of detectingthe applied external force on the basis of the voltage V which is outputfrom the conversion and output circuit 20 and the compensation signalVoff which is output from the compensation signal output circuit 30. Theexternal force detection circuit 40 a includes an amplifier 41 aconnected to the conversion and output circuit 20, an amplifier 42 aconnected to the compensation signal output circuit 30, and adifferential amplifier 43 a connected to the amplifiers 41 a and 42 a.

An input terminal of the amplifier 41 a is connected to the outputterminal of the operational amplifier 21 of the conversion and outputcircuit 20, and an output terminal of the amplifier 41 a is connected toa first input terminal (negative input) of the differential amplifier 43a. An input terminal of the amplifier 42 a is connected to the outputterminal of the operational amplifier 31 of the compensation signaloutput circuit 30, and an output terminal of the amplifier 42 a isconnected to a second input terminal (positive input) of thedifferential amplifier 43 a.

The amplifier 41 a has a function of giving a gain G=a to the voltage Vwhich is output from the conversion and output circuit 20, andperforming correction. The amplifier 42 a has a function of giving again G=b to the compensation signal Voff which is output from thecompensation signal output circuit 30, and performing correction.

It is preferable that a gain factor a of the amplifier 41 a and a gainfactor b of the amplifier 42 a satisfy the relational expression ofa=C1/C2×b. Here, C1 is the capacitance of the capacitor 22 of theconversion and output circuit 20, and C2 is the capacitance of thecapacitor 32 of the compensation signal output circuit 30. Thereby, itis possible to correct the sensitivity difference between the voltage Vand the compensation signal Voff which is caused by the differencebetween the capacitance C1 of the capacitor 22 and the capacitance C2 ofthe capacitor 32. As a result, the values of the corrected output driftD (that is, a×D) and the corrected compensation signal Voff (that is,b×D) become substantially equal to each other. The phrase “substantiallyequal” as used herein means that the difference between the correctedoutput drift D (that is, a×D) and the corrected compensation signal Voff(that is, b×D) is sufficiently small to the extent of being negligible.Meanwhile, a=1 means that the voltage V is not corrected. Similarly, b=1means that the compensation signal Voff is not corrected.

The differential amplifier 43 a has a function of taking the differencebetween the voltage V corrected by the amplifier 41 a and thecompensation signal Voff corrected by the amplifier 42 a, and outputtinga signal F. As described above, the values of the output drift D,included in the corrected voltage V, which is caused by the leakagecurrent and the corrected compensation signal Voff are substantiallyequal to each other. Therefore, the signal F which is output from anoutput terminal of the differential amplifier 43 a is as follows.

$\begin{matrix}{F = {{a \times V} - {b \times {Voff}}}} \\{= {{a \times \left( {{Vt} + D} \right)} - {b \times {Voff}}}} \\{= {{a \times {Vt}} + {a \times D} - {b \times {Voff}}}} \\{\approx {a \times {Vt}}}\end{matrix}$

In this manner, the difference between the corrected voltage V and thecorrected compensation signal Voff is taken, and thus it is possible toreduce (remove) the output drift D caused by the leakage current fromthe corrected voltage V. The external force detection circuit 40 a hassuch a configuration, and thus can output the signal F proportional tothe accumulated amount of the charge Q which is output from the chargeoutput element 10 a. Since the signal F corresponds to an external forceapplied to the charge output element 10 a, the force detection device 1a can detect the external force applied to the charge output element 10a.

In this manner, the force detection device 1 a of the embodimentincludes the compensation signal output circuit 30 and the externalforce detection circuit 40 a, and thus can reduce the output drift Dcaused by the leakage current of the switching element 23 of theconversion and output circuit 20. As a result, it is possible to improvethe detection accuracy and detection resolution of the force detectiondevice 1 a. In addition, a method of reducing the above-mentioned outputdrift D is effective even when the measurement time gets longer, andthus it is possible to lengthen the measurement time of the forcedetection device 1 a.

Second Embodiment

Next, a second embodiment of the invention will be described withreference to FIGS. 4 and 5. Hereinafter, the second embodiment will bedescribed with an emphasis on the difference with the above-mentionedfirst embodiment, and the description of the same particulars will beomitted.

FIG. 4 is a circuit diagram schematically illustrating the secondembodiment of the force detection device according to the invention.FIG. 5 is a cross-sectional view schematically illustrating a chargeoutput element of the force detection device shown in FIG. 4.

A force detection device 1 b shown in FIG. 4 has a function of detectingexternal forces applied along three axes (α (X)-axis, β (Y)-axis, and γ(Z)-axis) orthogonal to each other. The force detection device 1 bincludes a charge output element 10 b that outputs three charges Qx, Qy,and Qz in accordance with each external force applied (received) alongthree axes orthogonal to each other, a conversion and output circuit 20a that converts the charge Qx which is output from the charge outputelement 10 b into a voltage Vx, a conversion and output circuit 20 bthat converts the charge Qz which is output from the charge outputelement 10 b into a voltage Vz, a conversion and output circuit 20 cthat converts the charge Qy which is output from the charge outputelement 10 b into a voltage Vy, a compensation signal output circuit 30that outputs the compensation signal Voff, and an external forcedetection circuit 40 b that detects the applied external force.

Charge Output Element

The charge output element 10 b has a function of outputting threecharges Qx, Qy, and Qz in accordance with each external force applied(received) along three axes orthogonal to each other. As shown in FIG.5, the charge output element 10 b includes four ground electrode layers11 grounded to a ground (reference potential point) GND, a firstpiezoelectric substance 12 as a piezoelectric substance that outputs thecharge Qy in accordance with the external force (shearing force)parallel or substantially parallel to the β-axis, a second piezoelectricsubstance 13 that outputs the charge Qz in accordance with the externalforce (compressive/tensile force) parallel or substantially parallel tothe γ-axis, and a third piezoelectric substance 14 that outputs thecharge Qx in accordance with the external force (shearing force)parallel or substantially parallel to the α-axis, and the groundelectrode layers 11 and each of the piezoelectric substances 12, 13, and14 are alternately laminated. Meanwhile, in FIG. 5, the laminationdirection of the ground electrode layers 11 and the piezoelectricsubstances 12, 13, and 14 is set to a γ-axis direction, and thedirections which are orthogonal to the γ-axis direction and areorthogonal to each other are set to an α-axis direction and a β-axisdirection, respectively.

In the shown configuration, the first piezoelectric substance 12, thesecond piezoelectric substance 13, and the third piezoelectric substance14 are laminated in this order from the lower side in FIG. 5, but theinvention is not limited thereto. The lamination order of thepiezoelectric substances 12, 13, and 14 is arbitrary.

The first piezoelectric substance 12 has a function of outputting thecharge Qy in accordance with the external force (shearing force)parallel or substantially parallel to the β-axis. The firstpiezoelectric substance 12 has the same structure and function as thoseof the piezoelectric substance 12 of the above-mentioned firstembodiment.

The second piezoelectric substance 13 has a function of outputting thecharge Qz in accordance with the external force (compressive/tensileforce) applied (received) along the γ-axis. The second piezoelectricsubstance 13 is configured to output positive charge in accordance withthe compressive force parallel or substantially parallel to the γ-axis,and to output negative charge in accordance with the tensile forceparallel or substantially parallel to the γ-axis.

The second piezoelectric substance 13 includes a third piezoelectricplate 231 having a third crystal axis CA3, a fourth piezoelectric plate233, provided facing the third piezoelectric plate 231, which has afourth crystal axis CA4, and an internal electrode 232, provided betweenthe third piezoelectric plate 231 and the fourth piezoelectric plate233, which outputs the charge Qz.

The third piezoelectric plate 231 is constituted by a piezoelectricsubstance having the third crystal axis CA3 oriented in the positivedirection of the γ-axis. When the compressive force parallel orsubstantially parallel to the γ-axis is applied to the surface of thethird piezoelectric plate 231, charge is induced into the thirdpiezoelectric plate 231 by a piezoelectric effect. As a result, positivecharge is collected in the vicinity of the surface of the thirdpiezoelectric plate 231 on the internal electrode 232 side, and negativecharge is collected in the vicinity of the surface of the thirdpiezoelectric plate 231 on the ground electrode layer 11 side.Similarly, when the tensile force in the direction of the γ-axis isapplied to the surface of the third piezoelectric plate 231, negativecharge is collected in the vicinity of the surface of the thirdpiezoelectric plate 231 on the internal electrode 232 side, and positivecharge is collected in the vicinity of the surface of the thirdpiezoelectric plate 231 on the ground electrode layer 11 side.

The fourth piezoelectric plate 233 is constituted by a piezoelectricsubstance having the fourth crystal axis CA4 oriented in the negativedirection of the γ-axis. When the compressive force parallel orsubstantially parallel to the γ-axis is applied to the surface of thefourth piezoelectric plate 233, charge is induced into the fourthpiezoelectric plate 233 by a piezoelectric effect. As a result, positivecharge is collected in the vicinity of the surface of the fourthpiezoelectric plate 233 on the internal electrode 232 side, and negativecharge is collected in the vicinity of the surface of the fourthpiezoelectric plate 233 on the ground electrode layer 11 side.Similarly, when the tensile force parallel or substantially parallel tothe γ-axis is applied to the surface of the fourth piezoelectric plate233, negative charge is collected in the vicinity of the surface of thefourth piezoelectric plate 233 on the internal electrode 232 side, andpositive charge is collected in the vicinity of the surface of thefourth piezoelectric plate 233 on the ground electrode layer 11 side.

As constituent materials of the third piezoelectric plate 231 and thefourth piezoelectric plate 233, the same constituent materials as thoseof the first piezoelectric plate 121 and the second piezoelectric plate123 can be used. In addition, the piezoelectric plate, such as the thirdpiezoelectric plate 231 and the fourth piezoelectric plate 233, whichgenerates charge by the external force (compressive/tensile force)perpendicular to the surface direction of the layer can be formed of Xcut quartz crystal.

The internal electrode 232 has a function of outputting positive chargeor negative charge, generated within the third piezoelectric plate 231and the fourth piezoelectric plate 233, as the charge Qz. As describedabove, when the compressive force parallel or substantially parallel tothe γ-axis is applied to the surface of the third piezoelectric plate231 or the surface of the fourth piezoelectric plate 233, positivecharge is collected in the vicinity of the internal electrode 232. As aresult, positive charge Qz is output from the internal electrode 232. Onthe other hand, when the tensile force parallel or substantiallyparallel to the γ-axis is applied to the surface of the thirdpiezoelectric plate 231 or the surface of the fourth piezoelectric plate233, negative charge is collected in the vicinity of the internalelectrode 232. As a result, negative charge Qz is output from theinternal electrode 232.

The third piezoelectric substance 14 has a function of outputting thecharge Qx in accordance with the external force (shearing force) applied(received) along the α-axis. The third piezoelectric substance 14 isconfigured to output positive charge in accordance with an externalforce applied along the positive direction of the α-axis, and to outputnegative charge in accordance with an external force applied along thenegative direction of the α-axis.

The third piezoelectric substance 14 includes a fifth piezoelectricplate 241 having a fifth crystal axis CA5, a sixth piezoelectric plate243, provided facing the fifth piezoelectric plate 241, which has asixth crystal axis CA6, and an internal electrode 242, provided betweenthe fifth piezoelectric plate 241 and the sixth piezoelectric plate 243,which outputs the charge Qx.

The fifth piezoelectric plate 241 is constituted by a piezoelectricsubstance having the fifth crystal axis CA5 oriented in the negativedirection of the α-axis. When the external force along the positivedirection of the α-axis is applied to the surface of the fifthpiezoelectric plate 241, charge is induced into the fifth piezoelectricplate 241 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the fifth piezoelectricplate 241 on the internal electrode 242 side, and negative charge iscollected in the vicinity of the surface of the fifth piezoelectricplate 241 on the ground electrode layer 11 side. Similarly, when theexternal force along the negative direction of the α-axis is applied tothe surface of the fifth piezoelectric plate 241, negative charge iscollected in the vicinity of the surface of the fifth piezoelectricplate 241 on the internal electrode 242 side, and positive charge iscollected in the vicinity of the surface of the fifth piezoelectricplate 241 on the ground electrode layer 11 side.

The sixth piezoelectric plate 243 is constituted by a piezoelectricsubstance having the sixth crystal axis CA6 oriented in the positivedirection of the α-axis. When the external force along the positivedirection of the α-axis is applied to the surface of the sixthpiezoelectric plate 243, charge is induced into the sixth piezoelectricplate 243 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the sixth piezoelectricplate 243 on the internal electrode 242 side, and negative charge iscollected in the vicinity of the surface of the sixth piezoelectricplate 243 on the ground electrode layer 11 side. Similarly, when theexternal force along the negative direction of the α-axis is applied tothe surface of the sixth piezoelectric plate 243, negative charge iscollected in the vicinity of the surface of the sixth piezoelectricplate 243 on the internal electrode 242 side, and positive charge iscollected in the vicinity of the surface of the sixth piezoelectricplate 243 on the ground electrode layer 11 side.

As constituent materials of the fifth piezoelectric plate 241 and thesixth piezoelectric plate 243, the same constituent materials as thoseof the first piezoelectric plate 121 and the second piezoelectric plate123 can be used. In addition, the piezoelectric plate, such as the fifthpiezoelectric plate 241 and the sixth piezoelectric plate 243, whichgenerates charge by the external force (shearing force) applied alongthe surface direction of the layer can be formed of Y cut quartzcrystal, similarly to the first piezoelectric plate 121 and the secondpiezoelectric plate 123.

The internal electrode 242 has a function of outputting positive chargeor negative charge, generated within the fifth piezoelectric plate 241and the sixth piezoelectric plate 243, as the charge Qx. As describedabove, when the external force along the positive direction of theα-axis is applied to the surface of the fifth piezoelectric plate 241 orthe surface of the sixth piezoelectric plate 243, positive charge iscollected in the vicinity of the internal electrode 242. As a result,positive charge Qx is output from the internal electrode 242. On theother hand, when the external force along the negative direction of theα-axis is applied to the surface of the fifth piezoelectric plate 241 orthe surface of the sixth piezoelectric plate 243, negative charge iscollected in the vicinity of the internal electrode 242. As a result,negative charge Qx is output from the internal electrode 242.

In this manner, the first piezoelectric substance 12, the secondpiezoelectric substance 13, and the third piezoelectric substance 14 arelaminated so that the force detection directions of the respectivepiezoelectric substances are orthogonal to each other. Thereby, each ofthe piezoelectric substances 12, 13, and 14 can induce charge inaccordance with force components orthogonal to each other. Therefore,the charge output element 10 b can output three charges Qx, Qy, and Qzin accordance with the respective external forces applied along threeaxes (α (X)-axis, β (Y)-axis, and γ (Z)-axis).

In addition, the amount of charge generation per unit force of the firstpiezoelectric substance 12 and the third piezoelectric substance 14which are formed of Y cut quartz crystal is, for example, 8 pC/N. Theamount of charge generation per unit force of the second piezoelectricsubstance 13 formed of X cut quartz crystal is, for example, 4 pC/N.Therefore, generally, the sensitivity of the charge output element 10 bto the external force (compressive/tensile force) parallel orsubstantially parallel to the γ-axis is lower than the sensitivity ofthe charge output element 10 b to the external force (shearing force)parallel or substantially parallel to the α-axis or the β-axis. For thisreason, generally, the charge Qz which is output from the secondpiezoelectric substance 13 is smaller than the charge Qy which is outputfrom the first piezoelectric substance 12 and the charge Qx which isoutput from the third piezoelectric substance 14.

Conversion and Output Circuit

The conversion and output circuits 20 a and 20 c have the sameconfiguration as that of the conversion and output circuit 20 of thefirst embodiment. The conversion and output circuit 20 b has the sameconfiguration as that of the conversion and output circuit 20 of thefirst embodiment, except for capacitance C3 of the capacitor 22. Theconversion and output circuit 20 a has a function of converting thecharge Qx which is output from the charge output element 10 b into thevoltage Vx. The conversion and output circuit 20 b has a function ofconverting the charge Qz which is output from the charge output element10 b into the voltage Vz. The conversion and output circuit 20 c has afunction of converting the charge Qy which is output from the chargeoutput element 10 b into the voltage Vy.

The same drive circuit may be connected to the switching element 23 ofeach of the conversion and output circuits 20 a, 20 b, and 20 c, anddifferent drive circuits may be connected thereto. All the on/offsignals synchronized from the drive circuits are input to the respectiveswitching elements 23. Thereby, the operations of the switching elements23 of the respective conversion and output circuits 20 a, 20 b, and 20 care synchronized with each other. That is, the on/off timings of theswitching elements 23 of the respective conversion and output circuits20 a, 20 b, and 20 c are consistent with each other.

As described above, generally, the charge Qz which is output from thesecond piezoelectric substance 13 is smaller than the charge Qy which isoutput from the first piezoelectric substance 12 and the charge Qx whichis output from the third piezoelectric substance 14. Therefore, it ispreferable that the capacitance C3 of the capacitor 22 of the conversionand output circuit 20 b is smaller than the capacitance C1 of thecapacitor 22 of the conversion and output circuits 20 a and 20 c.Thereby, the charge Qz can be converted into a voltage accurately.

In addition, the capacitance ratio C3/C1 of the capacitance C3 to thecapacitance C1 is preferably 0.3 to 0.8, and is more preferably 0.45 to0.6. When the capacitance ratio C3/C1 falls below the lower limit, thecapacitor 22 may be saturated by the charge Qz. On the other hand, whenthe capacitance ratio C3/C1 exceeds the upper limit, sufficientsensitivity to the charge Qz may not be obtained.

In addition, since the switching elements 23 of the respectiveconversion and output circuits 20 a, 20 b, and 20 c are the samesemiconductor switching element, and are mounted under the same useenvironment, the leakage currents of the respective switching elements23 are substantially equal to each other. Therefore, the output drifts Dof the respective switching elements 23 are also substantially equal toeach other.

The phrase “under the same use environment” as mentioned above includes,for example, a case where the respective switching elements 23 aremounted in the vicinity of each other, a case where the respectiveswitching elements 23 are mounted in the same housing, a case where therespective switching elements 23 are mounted on the same semiconductorsubstrate, and the like.

Among these cases, it is preferable that the respective switchingelements 23 are mounted on the same semiconductor substrate. Therespective switching elements 23 are mounted on the same semiconductorsubstrate, thereby allowing the temperatures and humidities around therespective switching elements 23 to be made substantially equal to eachother. In addition, when the respective switching elements are mountedon the same semiconductor substrate, the respective switching elements23 can be formed in the same process, which leads to the advantage ofshortening a working process. In addition, since the respectiveswitching elements 23 can be formed in the same process, it is possibleto suppress a variation in the characteristics of the respectiveswitching elements 23. Therefore, the leakage currents caused by thesemiconductor structures of the respective switching elements 23 can bemade equal to each other with a higher degree of accuracy.

Compensation Signal Output Circuit

The compensation signal output circuit 30 has the same configuration asthat of the compensation signal output circuit 30 of the firstembodiment. The compensation signal output circuit 30 has a function ofoutputting the compensation signal Voff for compensating for the voltageVx which is output from the conversion and output circuit 20 a, thevoltage Vz which is output from the conversion and output circuit 20 b,and the voltage Vy which is output from the conversion and outputcircuit 20 c. As shown in the drawing, the compensation signal outputcircuit 30 may be provided independently of the conversion and outputcircuits 20 a, 20 b, and 20 c.

In addition, the switching element 33 of the compensation signal outputcircuit 30 is mounted under the same use environment as that of theswitching element 23 of each of the conversion and output circuits 20 a,20 b, and 20 c. Thereby, the leakage current of the switching element 33operates simultaneously with the leakage current of each switchingelement 23. Therefore, the compensation signal output circuit 30 detectsthe leakage current of the switching element 33, thereby allowing theleakage current of each switching element 23 to be indirectly acquired.The compensation signal output circuit 30 outputs the leakage current ofthe acquired switching element 33 as the compensation signal Voff.

In this manner, the compensation signal output circuit 30 detects theleakage current of the switching element 33, thereby allowing theleakage current of the switching element 23 of each of the conversionand output circuits 20 a, 20 b, and 20 c to be indirectly acquired.Therefore, the force detection device 1 b of the embodiment is notrequired to provide three leakage current detection circuits for use inthe respective conversion and output circuits 20 a, 20 b, and 20 c.Therefore, it is possible to reduce the number of circuits required forthe force detection device 1 b, and to reduce the size and weight of theforce detection device 1 b.

In addition, it is preferable that the capacitance C2 of the capacitor32 of the compensation signal output circuit 30 is smaller than thecapacitance C3 of the capacitor 22 of the conversion and output circuit20 b. That is, it is preferable that the magnitude relation between thecapacitances C1 and C3 of the capacitors 22 provided in the respectiveconversion and output circuits 20 a, 20 b, and 20 c and the capacitanceC2 of the capacitor 32 is C2<C3<C1. Thereby, the charges Qx, Qy, and Qzand the leakage current of the switching element 33 can be convertedinto voltages accurately.

External Force Detection Circuit

The external force detection circuit 40 b has a function of detecting anapplied external force on the basis of the voltage Vx which is outputfrom the conversion and output circuit 20 a, the voltage Vz which isoutput from the conversion and output circuit 20 b, the voltage Vy whichis output from the conversion and output circuit 20 c, and thecompensation signal Voff which is output from the compensation signaloutput circuit 30. The external force detection circuit 40 b includes anAD converter 41 b connected to the conversion and output circuits 20 a,20 b, and 20 c and the compensation signal output circuit 30, and anarithmetic operation circuit 42 b connected to the AD converter 41 b.

The AD converter 41 b has a function of converting the voltages Vx, Vy,and Vz and the compensation signal Voff from analog signals into digitalsignals. The voltages Vx, Vy, and Vz and the compensation signal Voffwhich are digitally converted by the AD converter 41 b are input to thearithmetic operation circuit 42 b.

The arithmetic operation circuit 42 b includes a gain correction portion(not shown) that gives gains to the voltages Vx, Vy, and Vz and thecompensation signal Voff, which are digitally converted, to performcorrection, and an arithmetic operation portion (not shown) thatarithmetically operates and outputs signals Fx, Fy, and Fz on the basisof the voltages Vx, Vy, and Vz and the compensation signal Voff whichare corrected by the gain correction portion.

The gain correction portion has a function of giving a gain G=a to thevoltages Vx and Vy, giving a gain G=c to the voltage Vz, and giving again G=b to the compensation signal Voff, to thereby perform thecorrection of the voltages Vx, Vy, and Vz and the compensation signalVoff. It is preferable that the gain factor a and the gain factor bsatisfy the relational expression of a=C1/C2×b. It is preferable thatthe gain factor c and the gain factor b satisfy the relationalexpression of c=C3/C2×b.

Herein, C1 is the capacitance of the capacitor 22 of the conversion andoutput circuits 20 a and 20 c, C2 is the capacitance of the capacitor 32of the compensation signal output circuit 30, and C3 is the capacitanceof the capacitor 22 of the conversion and output circuit 20 b. Thereby,it is possible to correct the sensitivity difference between thevoltages Vx and Vy and the compensation signal Voff which is caused bythe difference between the capacitance C1 of the capacitor 22 of theconversion and output circuits 20 a and 20 c and the capacitance C2 ofthe capacitor 32. Similarly, it is possible to correct the sensitivitydifference between the voltage Vz and the compensation signal Voff whichis caused by the difference between the capacitance C3 of the capacitor22 of the conversion and output circuit 20 b and the capacitance C2 ofthe capacitor 32. Thereby, the output drift D (that is, a×D or c×D),included in the corrected voltages Vx, Vy, and Vz, which is caused bythe leakage current and the corrected compensation signal Voff (that is,b×D) become substantially equal to each other. Meanwhile, a=1 means thatthe voltages Vx and Vy are not corrected. In addition, b=1 means thatthe compensation signal Voff is not corrected. Similarly, c=1 means thatthe voltage Vz is not corrected.

The arithmetic operation portion has a function of arithmeticallyoperating and outputting the signals Fx, Fy, and Fz on the basis of thevoltages Vx, Vy, and Vz corrected by the gain correction portion and thecompensation signal Voff corrected by the gain correction portion. Thesignal Fx is arithmetically operated by taking the difference betweenthe voltage Vx (that is, a×Vx) corrected by the gain correction portionand the compensation signal Voff (b×Voff) corrected by the gaincorrection portion. Therefore, the output signal Fx is as follows.

$\begin{matrix}{{Fx} = {{a \times {Vx}} - {b \times {Voff}}}} \\{= {{a \times \left( {{Vxt} + D} \right)} - {b \times {Voff}}}} \\{= {{a \times {Vxt}} + {a \times D} - {b \times {Voff}}}} \\{\approx {a \times {Vxt}}}\end{matrix}$

where Vxt is a voltage component (true value), included in the voltageVx, which is proportional to the accumulated amount of the charge Qx.

Similarly, the signal Fy is arithmetically operated by taking thedifference between the voltage Vy (that is, a×Vy) corrected by the gaincorrection portion and the compensation signal Voff (b×Voff) correctedby the gain correction portion. Therefore, the output signal Fy is asfollows.

$\begin{matrix}{{Fy} = {{a \times {Vy}} - {b \times {Voff}}}} \\{= {{a \times \left( {{Vyt} + D} \right)} - {b \times {Voff}}}} \\{= {{a \times {Vyt}} + {a \times D} - {b \times {Voff}}}} \\{\approx {a \times {Vyt}}}\end{matrix}$

where, Vyt is a voltage component (true value), included in the voltageVy, which is proportional to the accumulated amount of the charge Qy.

Similarly, the signal Fz is arithmetically operated by taking thedifference between the voltage Vz (that is, c×Vz) corrected by the gaincorrection portion and the compensation signal Voff (b×Voff) correctedby the gain correction portion. Therefore, the output signal Fz is asfollows.

$\begin{matrix}{{Fz} = {{c \times {Vz}} - {b \times {Voff}}}} \\{= {{c \times \left( {{Vzt} + D} \right)} - {b \times {Voff}}}} \\{= {{c \times {Vzt}} + {c \times D} - {b \times {Voff}}}} \\{\approx {c \times {Vzt}}}\end{matrix}$

where, Vzt is a voltage component (true value), included in the voltageVz, which is proportional to the accumulated amount of the charge Qz.

As described above, since the output drift D (that is, a×D or c×D),included in the corrected voltages Vx, Vy, and Vz, which is caused bythe leakage current and the corrected compensation signal Voff (b×Voff)are substantially equal to each other, it is possible to reduce (remove)the output drift D caused by the leakage current from the correctedvoltages Vx, Vy, and Vz.

The arithmetic operation circuit 42 b has such a configuration, and thuscan output the signals Fx, Fy, and Fz proportional to the accumulatedamounts of the charges Qx, Qy, and Qz which are output from the chargeoutput element 10 b. Since the signals Fx, Fy, and Fz correspond tothree-axis forces (shearing force and compressive/tensile force) appliedto the charge output element 10 b, the force detection device 1 b candetect the three-axis forces applied to the charge output element 10 a.

In this manner, the force detection device 1 b of the embodimentincludes the compensation signal output circuit 30 and the externalforce detection circuit 40 b, and thus can reduce the output drift Dcaused by the leakage current of the switching element 23 of theconversion and output circuits 20 a, 20 b, and 20 c. As a result, it ispossible to improve the detection accuracy and detection resolution ofthe force detection device 1 b. In addition, a method of reducing theabove-mentioned output drift D is effective even when the measurementtime gets longer, and thus it is possible to lengthen the measurementtime of the force detection device 1 b.

Third Embodiment

Next, a six-axis force detection device (force detection device) whichis a third embodiment of the invention will be described with referenceto FIG. 6. Hereinafter, the third embodiment will be described with anemphasis on the differences with the above-mentioned first and secondembodiments, and the description of the same particulars will beomitted.

FIG. 6 is a perspective view schematically illustrating the thirdembodiment of the force detection device according to the invention. Asix-axis force detection device (force detection device) 100 of FIG. 6has a function of detecting six-axis forces (translational forcecomponents in the directions of the x, y, and z axes and rotationalforce components around the x, y, and z axes). The six-axis forcedetection device 100 includes a first substrate 101, a second substrate102 facing the first substrate 101, four force detection devices 1 binterposed (provided) between the first substrate 101 and the secondsubstrate 102, and an arithmetic operation portion (not shown) connectedto the four force detection devices 1 b. Meanwhile, in FIG. 6, thesecond substrate 102 is shown in perspective for convenience ofdescription.

As described above, the force detection device 1 b has a function ofdetecting external forces applied along three axes (α(X)-axis, β(Y)-axis, and γ(Z)-axis) orthogonal to each other. In addition, theforce detection devices 1 b are interposed (provided) between the firstsubstrate 101 and the second substrate 102 with all facing the samedirection. As shown in the drawing, the force detection devices 1 b arepreferably disposed at equal angular intervals along the circumferentialdirection of the first substrate 101 or the second substrate 102, andare more preferably disposed at equal intervals concentrically about thecentral point of the first substrate 101 or the second substrate 102. Inthis manner, the force detection devices 1 b are disposed, and thus itis possible to detect the external forces in an unbiased manner.

When an external force by which the relative positions of the firstsubstrate 101 and the second substrate 102 are mutually shifted in anFx0 direction is applied, the force detection devices 1 b output signalsFx1, Fx2, Fx3, and Fx4, respectively. Similarly, when an external forceby which the relative positions of the first substrate 101 and thesecond substrate 102 are mutually shifted in an Fy0 direction isapplied, the force detection devices 1 b output signals Fy1, Fy2, Fy3,and Fy4, respectively. In addition, when an external force by which therelative positions of the first substrate 101 and the second substrate102 are mutually shifted in an Fz0 direction is applied, the forcedetection devices 1 b output signals Fz1, Fz2, Fz3, and Fz4,respectively.

In addition, in the first substrate 101 and the second substrate 102,the relative displacement of rotation about the x-axis, the relativedisplacement of rotation about the y-axis, and the relative displacementof rotation about the z-axis can be made with each other, and anexternal force associated with each rotation can be transmitted to theforce detection device 1 b.

The arithmetic operation portion has a function of arithmeticallyoperating a translational force component Fx0 in the x-axis direction, atranslational force component Fy0 in the y-axis direction, atranslational force component Fz0 in the z-axis direction, a rotationalforce component Mx about the x-axis, a rotational force component Myabout the y-axis, and a rotational force component Mz about the z-axis,on the basis of a signal which is output from each of the forcedetection devices 1 b. The force components can be obtained by thefollowing expressions, respectively.

Fx0=Fx1+Fx2+Fx3+Fx4

Fy0=Fy1+Fy2+Fy3+Fy4

Fz0=Fz1+Fz2+Fz3+Fz4

Mx=b×(Fz4−Fz2)

My=a×(Fz3−Fz1)

Mz=b×(Fx2−Fx4)+a×(Fy1−Fy3)

Herein, a and b are constants.

In this manner, the six-axis force detection device 100 includes thefirst substrate 101, the second substrate 102, a plurality of forcedetection devices 1 b and the arithmetic operation portion, and thus candetect six-axis forces.

Meanwhile, in the shown configuration, the number of force detectiondevices 1 b is four, but the invention is not limited thereto. When thesix-axis force detection device 100 has at least three force detectiondevices 1 b, six-axis forces can be detected. When the number of forcedetection devices 1 b is three, the number of force detection devices 1b is small, and thus it is possible to reduce the weight of the six-axisforce detection device 100. When the number of force detection devices 1b is four as shown in the drawing, six-axis forces can be obtained by avery simple arithmetic operation as described above, and thus it ispossible to simplify the arithmetic operation portion. In addition, whenthe number of force detection devices 1 b is six, it is possible todetect six-axis forces with a higher degree of accuracy.

Fourth Embodiment

FIG. 7A is a perspective view schematically illustrating a fourthembodiment of the force detection device according to the invention.FIG. 7B is a plan view schematically illustrating the fourth embodimentof the force detection device according to the invention. Meanwhile, inFIG. 7A, some of the components are shown in perspective for the purposeof description. In FIG. 7B, some of the components are omitted for thepurpose of description. FIG. 8 is a circuit diagram schematicallyillustrating the force detection device shown in FIGS. 7A and 7B. FIG. 9is a cross-sectional view schematically illustrating a charge outputelement of the force detection device shown in FIGS. 7A and 7B.

A force detection device 101 a shown in FIGS. 7A and 7B has a functionof detecting shearing forces (external forces applied along the x-axisand the y-axis in FIGS. 7A and 7B). The force detection device 101 aincludes a base plate 2, a cover plate 4 provided separately so as toface the base plate 2, force detection elements 103 a and 103 b,interposed (provided) between the base plate 2 and the cover plate 4,which output voltages in accordance with external forces, and anexternal force detection circuit 105 (not shown in FIG. 7A or 7B; seeFIG. 8) that detects the external forces on the basis of the voltagewhich is output from each of the force detection elements 103 a and 103b. Here, the force detection element 103 a is equivalent to a firstelement, and the force detection element 103 b is equivalent to a secondelement.

Force Detection Element

The force detection elements 103 a and 103 b shown in FIGS. 7A and 7Bhave a function of outputting a voltage V in accordance with the appliedshearing forces (external forces applied along the x-axis and the y-axisin FIGS. 7A and 7B).

As shown in FIG. 8, each of the force detection elements 103 a and 103 bincludes a charge output element 131 that outputs charge Q in accordancewith the applied shearing force and a conversion and output circuit 132that converts the charge Q which is output from the charge outputelement 131 into the voltage V. Specifically, the force detectionelement 103 a includes the charge output element 131 that outputs chargeQ1 and the conversion and output circuit 132 that converts the charge Q1into a voltage V1 and outputs the resultant. In addition, the forcedetection element 103 b includes the charge output element 131 thatoutputs charge Q2 and the conversion and output circuit 132 thatconverts the charge Q2 into a voltage V2 and outputs the resultant.

Charge Output Element

The charge output element 131 shown in FIG. 9 has a function ofoutputting the charge Q in accordance with an external force (shearingforce) parallel or substantially parallel to the β-axis in FIG. 9. Thecharge output element 131 includes two ground electrode layers 310 and aβ-axis piezoelectric substance 320 provided between the two groundelectrode layers 310. Meanwhile, in FIG. 9, the lamination direction ofthe ground electrode layers 310 and the β-axis piezoelectric substance320 is set to a γ-axis direction, and the directions which areorthogonal to the γ-axis direction and are orthogonal to each other areset to an α-axis direction and a β-axis direction, respectively.

In the shown configuration, both the ground electrode layer 310 and theβ-axis piezoelectric substance 320 have the same width (length in ahorizontal direction in the drawing), but the invention is not limitedthereto. For example, the width of the ground electrode layer 310 may begreater than the width of the β-axis piezoelectric substance 320, orvice versa.

The ground electrode layer 310 is an electrode grounded to a ground(reference potential point) GND. Materials constituting the groundelectrode layer 310, though not particularly limited are preferably, forexample, gold, chromium, titanium, aluminum, copper, iron or an alloycontaining these materials.

The β-axis piezoelectric substance 320 has a function of outputting thecharge Q in accordance with the external force (shearing force) parallelor substantially parallel to the β-axis. The β-axis piezoelectricsubstance 320 is configured to output positive charge in accordance withan external force applied along the positive direction of the β-axis,and to output negative charge in accordance with an external forceapplied along the negative direction of the β-axis. That is, the β-axispiezoelectric substance 320 has an electric axis Pβ facing the positivedirection of the β-axis.

The β-axis piezoelectric substance 320 includes a first piezoelectricplate 321 having a first crystal axis CA1, a second piezoelectric plate323, provided facing the first piezoelectric plate 321, which has asecond crystal axis CA2, and an internal electrode 322, provided betweenthe first piezoelectric plate 321 and the second piezoelectric plate323, which outputs the charge Q. In addition, the lamination order ofrespective layers constituting the β-axis piezoelectric substance 320 isthe first piezoelectric plate 321, the internal electrode 322, and thesecond piezoelectric plate 323 in this order from the lower side in FIG.9.

The first piezoelectric plate 321 is constituted by a piezoelectricsubstance having the first crystal axis CA1 oriented in the negativedirection of the β-axis. When the external force along the positivedirection of the β-axis is applied to the surface of the firstpiezoelectric plate 321, charge is induced into the first piezoelectricplate 321 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the first piezoelectricplate 321 on the internal electrode 322 side, and negative charge iscollected in the vicinity of the surface of the first piezoelectricplate 321 on the ground electrode layer 310 side. Similarly, when theexternal force along the negative direction of the β-axis is applied tothe surface of the first piezoelectric plate 321, negative charge iscollected in the vicinity of the surface of the first piezoelectricplate 321 on the internal electrode 322 side, and positive charge iscollected in the vicinity of the surface of the first piezoelectricplate 321 on the ground electrode layer 310 side.

The second piezoelectric plate 323 is constituted by a piezoelectricsubstance having the second crystal axis CA2 oriented in the positivedirection of the β-axis. When the external force along the positivedirection of the β-axis is applied to the surface of the secondpiezoelectric plate 323, charge is induced into the second piezoelectricplate 323 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the second piezoelectricplate 323 on the internal electrode 322 side, and negative charge iscollected in the vicinity of the surface of the second piezoelectricplate 323 on the ground electrode layer 310 side. Similarly, when theexternal force along the negative direction of the β-axis is applied tothe surface of the second piezoelectric plate 323, negative charge iscollected in the vicinity of the surface of the second piezoelectricplate 323 on the internal electrode 322 side, and positive charge iscollected in the vicinity of the surface of the second piezoelectricplate 323 on the ground electrode layer 310 side.

In this manner, the direction of the first crystal axis CA1 of the firstpiezoelectric plate 321 is opposite to the direction of the secondcrystal axis CA2 of the second piezoelectric plate 323. Thereby, it ispossible to increase the positive charge or the negative chargecollected in the vicinity of the internal electrode 322, as comparedwith a case where the β-axis piezoelectric substance 320 is constitutedby only any one of the first piezoelectric plate 321 and the secondpiezoelectric plate 323, and the internal electrode 322. As a result, itis possible to increase the charge Q which is output from the internalelectrode 322.

Meanwhile, constituent materials of the first piezoelectric plate 321and the second piezoelectric plate 323 include quartz crystal, topaz,barium titanate, lead titanate, lead zirconate titanate (PZT:Pb(Zr,Ti)O₃), lithium niobate, lithium tantalite, and the like. Among thesematerials, particularly, quartz crystal is preferable. This is because apiezoelectric plate formed of quartz crystal has characteristicsexcellent in a wide dynamic range, high rigidity, high naturalfrequency, and high load bearing capacity. In addition, a piezoelectricplate, such as the first piezoelectric plate 321 and the secondpiezoelectric plate 323, which generates charge by an external force(shearing force) applied along the surface direction of the layer can beformed of Y cut quartz crystal.

The internal electrode 322 has a function of outputting positive chargeor negative charge, generated within the first piezoelectric plate 321and the second piezoelectric plate 323, as the charge Q. As describedabove, when the external force along the positive direction of theβ-axis is applied to the surface of the first piezoelectric plate 321 orthe surface of the second piezoelectric plate 323, positive charge iscollected in the vicinity of the internal electrode 322. As a result,positive charge Q is output from the internal electrode 322. On theother hand, when the external force along the negative direction of theβ-axis is applied to the surface of the first piezoelectric plate 321 orthe surface of the second piezoelectric plate 323, negative charge iscollected in the vicinity of the internal electrode 322. As a result,negative charge Q is output from the internal electrode 322.

In this manner, the charge output element 131 includes the groundelectrode layer 310 and the β-axis piezoelectric substance 320 mentionedabove, and thus can output the charge Q in accordance with the externalforce (shearing force) parallel or substantially parallel to the β-axisin FIG. 9.

Meanwhile, an example has been described in which the charge outputelement 131 has a function of outputting the charge Q in accordance withthe external force (shearing force) parallel or substantially parallelto the β-axis, but the invention is not limited thereto. By using thefirst piezoelectric plate 321 having the orientation direction of thefirst crystal axis CA1 being the α-axis direction different from theβ-axis direction and the second piezoelectric plate 323 having theorientation direction of the second crystal axis CA2 being the α-axisdirection different from the β-axis direction, it is possible to formthe charge output element 131 that outputs the charge Q in accordancewith the external force (shearing force) parallel or substantiallyparallel to the α-axis. Such a case is also within the scope of theinvention.

Conversion and Output Circuit

The conversion and output circuit 132 has a function of converting thecharge Q (Q1, Q2) which is output from the charge output element 131into the voltage V (V1, V2). The conversion and output circuit 132includes an operational amplifier 133, a capacitor 134, and a switchingelement 135. A first input terminal (negative input) of the operationalamplifier 133 is connected to the internal electrode 322 of the chargeoutput element 131, and a second input terminal (positive input) of theoperational amplifier 133 is grounded to a ground (reference potentialpoint). In addition, an output terminal of the operational amplifier 133is connected to the external force detection circuit 105. The capacitor134 is connected between the first input terminal and the outputterminal of the operational amplifier 133. The switching element 135 isconnected between the first input terminal and the output terminal ofthe operational amplifier 133, and is connected in parallel with thecapacitor 134. In addition, the switching element 135 is connected to adrive circuit (not shown), and executes a switching operation inaccordance with an on/off signal from the drive circuit.

When the switching element 135 is turned off, the charge Q which isoutput from the charge output element 131 is accumulated in thecapacitor 134 having the capacitance C1, and is output to the externalforce detection circuit 105 as the voltage V. Next, when the switchingelement 135 is turned on, both terminals of the capacitor 134 areshort-circuited therebetween. As a result, the charge Q accumulated inthe capacitor 134 is discharged to be held at 0 coulombs, and thevoltage V which is output to the external force detection circuit 105 isheld at 0 volts. The turn-on of the switching element 135 refers to theresetting of the conversion and output circuit 132.

The switching element 135 is a semiconductor switching element such as aMOSFET (Metal Oxide Semiconductor Field Effect Transistor). Asemiconductor switching element is smaller in size and lighter in weightthan a mechanical switch, and thus has an advantage in reducing the sizeand weight of the force detection device 101 a. Hereinafter, as arepresentative example, a case where a MOSFET is used as the switchingelement 135 will be described.

The switching element 135 has a drain electrode, a source electrode, anda gate electrode. One of the drain electrode and the source electrode ofthe switching element 135 is connected to the first input terminal ofthe operational amplifier 133, and the other of the drain electrode andthe source electrode is connected to the output terminal of theoperational amplifier 133. In addition, a gate electrode of theswitching element 135 is connected to a drive circuit (not shown).

The voltage V which is output from the ideal conversion and outputcircuit 132 is proportional to the accumulated amount of the charge Qwhich is output from the charge output element 131. However, in theactual conversion and output circuit 132, a leakage current flowing fromthe switching element 135 into the capacitor 134 is generated. Such aleakage current acts as an output drift D included in the voltage V.Therefore, when a voltage component (true value) proportional to theaccumulated amount of the charge Q is set to Vt, the output voltage Vsatisfies the relation of V=Vt+D.

Since the output drift D is equivalent to an error on a measurementresult, there is a problem in that the detection accuracy and detectionresolution of the force detection elements 103 a and 103 b deterioratedue to the leakage current (output drift D). In addition, since theleakage current is accumulated in proportion to the measurement (drive)time, there is a problem in that the measurement time of the forcedetection device 101 a cannot be lengthened.

Such a leakage current is caused by semiconductor structures such as thelack of insulation properties of a gate insulating film, the refinementof a process rule, and the variation of impurity concentration in asemiconductor, and use environments such as temperature and humidity.The leakage current caused by the semiconductor structure serves as aneigenvalue for each switching element, and thus can be compensated forrelatively easily by measuring the leakage current caused by thesemiconductor structure in advance. However, since the leakage currentcaused by the use environment fluctuates depending on the useenvironment (condition), it is not likely that the leakage current willbe compensated for. The force detection device 101 a of the embodimentcan reduce the influence (output drift D) of the leakage current usingthe external force detection circuit 105 that detects an external force,on the basis of the voltages V1 and V2 which are output from the forcedetection elements 103 a and 103 b constituting an element pair and theforce detection elements 103 a and 103 b, respectively.

Next, the positional relation between the force detection elements 103 aand 103 b constituting an element pair will be described in detail withreference to FIGS. 7A and 7B. Meanwhile, in FIG. 7B, the cover plate 4is omitted for the purpose of description. In addition, in FIG. 7B, ahorizontal direction is set to an x-axis direction, and a directionorthogonal to the x-axis direction, that is, a vertical direction is setto a y-axis direction.

The force detection element 103 a has an electric axis Pβ1 along theabove-mentioned β-axis, and outputs the voltage V1 in accordance withthe external force (shearing force) applied along the β-axis. Similarly,the force detection element 103 b has a electric axis Pβ2 along theabove-mentioned β-axis, and outputs the voltage V2 in accordance withthe external force (shearing force) applied along the β-axis.

The force detection elements 103 a and 103 b are provided (interposed)between the base plate 2 and the cover plate 4. The electric axis Pβ1 ofthe force detection element 103 a has an angle θ1. Similarly, theelectric axis Pβ2 of the force detection element 103 b has an angle θ2.Meanwhile, the angles θ1 and θ2 are angles from the x-axis of areference coordinate system (x-axis and y-axis) of FIG. 7B.

As shown in FIG. 7B, the force detection elements 103 a and 103 b aredisposed so that the direction of the electric axis Pβ1 of the forcedetection element 103 a and the direction of the electric axis Pβ2 ofthe force detection element 103 b are different from each other, and areopposite to each other in the embodiment. The phrase “opposite to eachother” as used herein is not limited to a case where the direction ofthe electric axis Pβ1 and the direction of the electric axis Pβ2 faceeach other as shown in FIG. 7B, that is, a case where the angles θ1 andθ2 satisfy the relation of θ1=θ2. When at least the electric axis Pβ1and the electric axis Pβ2 are decomposed into a vector component in thex-axis direction and a vector component in the y-axis direction,respectively, which are orthogonal to each other, the vector componentof the electric axis Pβ1 in the x-axis direction and the vectorcomponent of the electric axis Pβ2 in the x-axis direction may beopposite to each other in direction, or the vector component of theelectric axis Pβ1 in the y-axis direction and the vector component ofthe electric axis Pβ2 in the y-axis direction may be opposite to eachother in direction.

In addition, it is preferable that the force detection elements 103 aand 103 b are disposed so that the vector component of the electric axisPβ1 in the x-axis direction and the vector component of the electricaxis Pβ2 in the x-axis direction are opposite to each other indirection, and the vector component of the electric axis Pβ1 in they-axis direction and the vector component of the electric axis Pβ2 inthe y-axis direction are opposite to each other in direction, that is,the relation of |θ1−θ2|<π/2 is satisfied. Thereby, it is possible todetect shearing forces Fx and Fy described later. In the followingdescription, typically, a case will be described in which the forcedetection elements 103 a and 103 b are disposed so as to satisfy therelation of |θ1−θ2|<π/2.

In addition, it is more preferable that the force detection elements 103a and 103 b are disposed so that the direction of the electric axis Pβ1and the direction of the electric axis Pβ2 face each other, that is, therelation of θ1=θ2 is satisfied. Thereby, the external force detectioncircuit 105 described later can detect the shearing forces Fx and Fywhile further reducing the output drift D.

In addition, when the force detection elements are disposed so that thedirection of the electric axis Pβ1 of the force detection element 103 aand the direction of the electric axis Pβ2 of the force detectionelement 103 b are opposite to each other, the arrangement of the forcedetection elements 103 a and 103 b is not particularly limited. However,as shown in FIG. 7B, it is preferable that the force detection element103 a and the force detection element 103 b are disposed on the sameaxis. Thereby, the shearing forces (external forces applied along thex-axis and the y-axis in the drawing) applied to the base plate 2 or thecover plate 4 can be detected in an unbiased manner.

In addition, the electric axis Pβ1 of the force detection element 103 aand the electric axis Pβ2 of the force detection element 103 b in FIG.7B face the outside (centrifugal direction) of the base plate 2, but theinvention is not limited thereto. That is, when the force detectionelements are disposed so that the direction of the electric axis Pβ1 ofthe force detection element 103 a and the direction of the electric axisPβ2 of the force detection element 103 b are opposite to each other, theelectric axis Pβ1 of the force detection element 103 a and the electricaxis Pβ2 of the force detection element 103 b may face the centraldirection (centripetal direction) of the base plate 2.

When a voltage component (true value) proportional to the accumulatedamount of charge Q1 which is output from the charge output element 131of the force detection element 103 a is set to Vt1, and a voltagecomponent (true value) proportional to the accumulated amount of chargeQ2 which is output from the charge output element 131 of the forcedetection element 103 b is set to Vt2, the voltage V1 which is outputfrom the force detection element 103 a and the voltage V2 which isoutput from the force detection element 103 b are as follows.

V1=Vt1+D

V2=Vt2+D

Meanwhile, the switching element 135 of the force detection element 103a and the switching element 135 of the force detection element 103 b arethe same semiconductor switching element, and the leakage currentsthereof are substantially equal to each other. Therefore, the outputdrift D included in the voltage V1 and the output drift D included inthe voltage V2 are substantially equal to each other. The phrase“substantially equal” as used herein refers to the fact that when adifference between two values to be compared is taken, the difference isnegligibly small as compared with an original value.

In addition, since the force detection elements 103 a and 103 b aredisposed so that the direction of the electric axis Pβ1 of the forcedetection element 103 a and the direction of the electric axis Pβ2 ofthe force detection element 103 b are opposite to each other, the signof the voltage component Vt1 included in the voltage V1 and the sign ofthe voltage component Vt2 included in the voltage V2 are not consistentwith each other. For example, when the sign of the voltage component Vt1is positive, the sign of the voltage component Vt2 becomes negative.Similarly, when the sign of the voltage component Vt1 is negative, thesign of the voltage component Vt2 becomes positive. Therefore, when thedifference between the voltage V1 which is output from the forcedetection element 103 a and the voltage V2 which is output from theforce detection element 103 b is taken, the absolute value of thedifference between the voltage component Vt1 and the voltage componentVt2 does not decrease.

On the other hand, since the output drift D included in the voltage V1and the output drift D included in the voltage V2 are independent of thedirections of the electric axes Pβ1 and Pβ2, the sign of the outputdrift D included in the voltage V1 and the sign of the output drift Dincluded in the voltage V2 are consistent with each other. Therefore,when the difference between the voltage V1 which is output from theforce detection element 103 a and the voltage V2 which is output fromthe force detection element 103 b is taken, the absolute value of thedifference between the output drift D included in the voltage V1 and theoutput drift D included in the voltage V2 decreases.

External Force Detection Circuit

The external force detection circuit 105 has a function of detecting theshearing forces (external forces applied along the x-axis and the y-axisin the drawing) applied to the force detection device 101 a by takingthe difference between the voltage V1 which is output from the forcedetection element 103 a and the voltage V2 which is output from theforce detection element 103 b.

The external force detection circuit 105 can detect the shearing forcesFx and Fy applied to the force detection device 101 a by taking thedifference between the voltages V1 and V2 as follows.

$\begin{matrix}{{Fx} = {{V\; 1{\cos ({\theta 1})}} - {V\; 2{\cos ({\theta 2})}}}} \\{= {{V\; t\; 1\; {\cos ({\theta 1})}} - {V\; t\; 2{\cos ({\theta 2})}} + {D\left\{ {{\cos ({\theta 1})} - {\cos \left( {\theta 2} \right)}} \right\}}}} \\{{Fy} = {{V\; 1{\sin ({\theta 1})}} - {V\; 2{\sin ({\theta 2})}}}} \\{= {{V\; t\; 1\; {\sin ({\theta 1})}} - {V\; t\; 2{\sin ({\theta 2})}} + {D\left\{ {{\sin ({\theta 1})} - {\sin \left( {\theta 2} \right)}} \right\}}}}\end{matrix}$

In this manner, when the difference between the voltage V1 which isoutput from the force detection element 103 a and the voltage V2 whichis output from the force detection element 103 b is taken, the absolutevalue of the difference between the voltage component Vt1 and thevoltage component Vt2 does not decrease, but the absolute value of theoutput drift D decreases. Therefore, it is possible to reduce the outputdrift D. As a result, a detection error caused by the leakage current(output drift D) becomes relatively small, and thus it is possible toimprove the detection accuracy and detection resolution of the forcedetection device 101 a. In addition, a method of reducing theabove-mentioned output drift D is effective even when the measurementtime gets longer, and thus it is possible to lengthen the measurementtime of the force detection device 101 a.

Further, when the angles θ1 and θ2 satisfy the relation of θ1=θ2, thatis, when the force detection elements 103 a and 103 b are disposed sothat the direction of the electric axis Pβ1 and the direction of theelectric axis Pβ2 face each other, the calculation expressions of Fx andFy mentioned above are simplified as follows.

Fx=(Vt1−Vt2)cos(θ1)

Fy=(Vt1−Vt2)sin(θ1)

In this case, it is possible to remove (further reduce) the output driftD. As a result, it is possible to further improve the detection accuracyand detection resolution of the force detection device 101 a. Inaddition, it is possible to further lengthen the measurement time of theforce detection device 101 a.

In this manner, the force detection device 101 a of the embodimentincludes force detection elements 103 a and 103 b which are disposed sothat the direction of the electric axis Pβ1 of the force detectionelement 103 a and the direction of the electric axis Pβ2 of the forcedetection element 103 b are opposite to each other, and the externalforce detection circuit 105 that detects the shearing force applied tothe force detection device 101 a by taking the difference between thevoltage V1 which is output from the force detection element 103 a andthe voltage V2 which is output from the force detection element 103 b,and thus can reduce the output drift D caused by the leakage current ofthe switching element 135 of the conversion and output circuit 132. As aresult, it is possible to improve the detection accuracy and detectionresolution of the force detection device 101 a. In addition, the methodof reducing the above-mentioned output drift D is effective even whenthe measurement time gets longer, and thus it is possible to lengthenthe measurement time of the force detection device 101 a. Further, sincea circuit, such as a reverse bias circuit, for reducing the output driftD is not required in the force detection device 101 a of the embodiment,it is possible to reduce the size of the force detection device 101 a.

Meanwhile, the force detection device 101 a of the embodiment includes apair of force detection elements 103 a and 103 b, but the invention isnot limited thereto. The force detection device 101 a may includemultiple pairs of force detection elements 103 a and 103 b, and such acase is also within the scope of the invention.

Fifth Embodiment

Next, a fifth embodiment of the invention will be described withreference to FIGS. 10A and 10B, FIG. 11, and FIGS. 12A and 12B.Hereinafter, the fifth embodiment will be described with an emphasis onthe difference with the above-mentioned fourth embodiment, and thedescription of the same particulars will be omitted.

FIG. 10A is a perspective view schematically illustrating the fifthembodiment of the force detection device according to the invention.FIG. 10B is a plan view schematically illustrating the fifth embodimentof the force detection device according to the invention. FIG. 11 is acircuit diagram schematically illustrating the force detection deviceshown in FIGS. 10A and 10B. FIG. 12A is a cross-sectional viewschematically illustrating a charge output element of the first forcedetection device shown in FIGS. 10A and 10B. FIG. 12B is across-sectional view schematically illustrating a charge output elementof the second force detection device shown in FIGS. 10A and 10B.Meanwhile, in FIG. 10A, some of the components are shown in perspectivefor the purpose of description, and in FIG. 10B, some of the componentsare omitted for the purpose of description.

A force detection device 101 b shown in FIGS. 10A and 10B has a functionof detecting six-axis forces (translational force components in thedirections of the x, y, and z axes and rotational force componentsaround the x, y, and z axes). The force detection device 101 b includesa base plate 2, a cover plate 4 provided separately so as to face thebase plate 2, force detection elements 30 a, 30 b, 30 c, and 30 d,provided (interposed) between the base plate 2 and the cover plate 4,which output voltages Vα, Vβ, and Vγ in accordance with external forces,and an external force detection circuit 50 (not shown in FIG. 10A or10B; see FIG. 11) that detects the six-axis forces on the basis of thevoltages Vα, Vβ, and Vγ which are output from the force detectionelements 30 a, 30 b, 30 c, and 30 d, respectively.

Force Detection Element

The force detection elements 30 a, 30 b, 30 c, and 30 d have functionsof outputting the voltages Vα, Vβ, and Vγ in accordance with therespective external forces applied along three axes (α-axis, β-axis, andγ-axis) orthogonal to each other. In addition, the force detectionelements 30 a and 30 c constitute a first element pair, and the forcedetection elements 30 b and 30 d constitute a second element pair. Theforce detection elements 30 a and 30 c belonging to the first elementpair have the same configuration. The force detection elements 30 b and30 d belonging to the second element pair have the same configuration.

As shown in FIG. 11, the force detection elements 30 a and 30 cbelonging to the first element pair include a first charge outputelement 301 a that outputs charges Qα, Qβ, and Qγ in accordance with theexternal forces applied along the three axes (α-axis, β-axis, andγ-axis) orthogonal to each other, a conversion and output circuit 132 athat converts the charge Qα which is output from the first charge outputelement 301 a into the voltage Vα, a conversion and output circuit 132 bthat converts the charge Qγ which is output from the first charge outputelement 301 a into the voltage Vγ, and a conversion and output circuit132 c that converts the charge Qβ which is output from the first chargeoutput element 301 a into the voltage Vβ. The force detection elements30 b and 30 d belonging to the second element pair have the sameconfiguration as that of the force detection elements 30 a and 30 cbelonging to the first element pair, except that each of the forcedetection elements includes a second charge output element 301 b havinga structure different from that of the first charge output element 301a.

Charge Output Element

The first charge output element 301 a shown in FIG. 12A has a functionof outputting the charges Qα, Qβ, and Qγ in accordance with therespective external forces applied along the three axes (α-axis, β-axis,and γ-axis) orthogonal to each other in FIGS. 12A and 12B. As shown inFIG. 12A, the first charge output element 301 a includes four groundelectrode layers 310 grounded to a ground (reference potential point)GND, a β-axis piezoelectric substance 320 that outputs the charge Qβ inaccordance with an external force (shearing force) parallel orsubstantially parallel to the β-axis, a first γ-axis piezoelectricsubstance 330 that outputs the charge Qγ in accordance with an externalforce (compressive/tensile force) parallel or substantially parallel tothe γ-axis, and a first α-axis piezoelectric substance 340 that outputsthe charge Qα in accordance with an external force (shearing force)parallel or substantially parallel to the α-axis, and the groundelectrode layers 310 and each of the piezoelectric substances 320, 330,and 340 are alternately laminated. Meanwhile, in FIGS. 12A and 12B, thelamination direction of the ground electrode layers 310 and each of thepiezoelectric substances 320, 330, and 340 is set to a γ-axis direction,and the directions which are orthogonal to the γ-axis direction and areorthogonal to each other are set to an α-axis direction and a β-axisdirection, respectively.

In the shown configuration, the β-axis piezoelectric substance 320, thefirst γ-axis piezoelectric substance 330, and the first α-axispiezoelectric substance 340 are laminated in this order from the lowerside in FIGS. 12A and 12B, but the invention is not limited thereto. Thelamination order of the piezoelectric substances 320, 330, and 340 isarbitrary.

The β-axis piezoelectric substance 320 has a function of outputting thecharge Qβ in accordance with the external force (shearing force)parallel or substantially parallel to the β-axis. The β-axispiezoelectric substance 320 has the same structure and function as thoseof the β-axis piezoelectric substance 320 of the above-mentioned fourthembodiment.

The first γ-axis piezoelectric substance 330 has a function ofoutputting the charge Qγ in accordance with the external force(compressive/tensile force) parallel or substantially parallel to theγ-axis. The first γ-axis piezoelectric substance 330 is configured tooutput positive charge in accordance with an external force appliedalong the positive direction of the γ-axis, and to output negativecharge in accordance with an external force applied along the negativedirection of the γ-axis. That is, the first γ-axis piezoelectricsubstance 330 has a electric axis Pγ facing the positive direction ofthe γ-axis in FIGS. 12A and 12B.

The first γ-axis piezoelectric substance 330 includes a thirdpiezoelectric plate 331 having a third crystal axis CA3, a fourthpiezoelectric plate 333, provided facing the third piezoelectric plate331, which has a fourth crystal axis CA4, and an internal electrode 332,provided between the third piezoelectric plate 331 and the fourthpiezoelectric plate 333, which outputs the charge Qγ. In addition, thelamination order of the respective layers constituting the first γ-axispiezoelectric substance 330 is the order of the third piezoelectricplate 331, the internal electrode 332, and the fourth piezoelectricplate 333 from the lower side in FIGS. 12A and 12B.

The third piezoelectric plate 331 is constituted by a piezoelectricsubstance having the third crystal axis CA3 oriented in the positivedirection of the γ-axis. When the external force along the positivedirection of the γ-axis is applied to the surface of the thirdpiezoelectric plate 331, charge is induced into the third piezoelectricplate 331 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the third piezoelectricplate 331 on the internal electrode 332 side, and negative charge iscollected in the vicinity of the surface of the third piezoelectricplate 331 on the ground electrode layer 310 side. Similarly, when theexternal force along the negative direction of the γ-axis is applied tothe surface of the third piezoelectric plate 331, negative charge iscollected in the vicinity of the surface of the third piezoelectricplate 331 on the internal electrode 332 side, and positive charge iscollected in the vicinity of the surface of the third piezoelectricplate 331 on the ground electrode layer 310 side.

The fourth piezoelectric plate 333 is constituted by a piezoelectricsubstance having the fourth crystal axis CA4 oriented in the negativedirection of the γ-axis. When the external force along the positivedirection of the γ-axis is applied to the surface of the fourthpiezoelectric plate 333, charge is induced into the fourth piezoelectricplate 333 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the fourth piezoelectricplate 333 on the internal electrode 332 side, and negative charge iscollected in the vicinity of the surface of the fourth piezoelectricplate 333 on the ground electrode layer 310 side. Similarly, when theexternal force along the negative direction of the γ-axis is applied tothe surface of the fourth piezoelectric plate 333, negative charge iscollected in the vicinity of the surface of the fourth piezoelectricplate 333 on the internal electrode 332 side, and positive charge iscollected in the vicinity of the surface of the fourth piezoelectricplate 333 on the ground electrode layer 310 side.

As constituent materials of the third piezoelectric plate 331 and thefourth piezoelectric plate 333, the same constituent materials as thoseof the first piezoelectric plate 321 and the second piezoelectric plate323 can be used. In addition, the piezoelectric plate, such as the thirdpiezoelectric plate 331 and the fourth piezoelectric plate 333, whichgenerates charge by the external force (compressive/tensile force)perpendicular to the surface direction of the layer can be formed of Xcut quartz crystal.

The internal electrode 332 has a function of outputting positive chargeor negative charge, generated within the third piezoelectric plate 331and the fourth piezoelectric plate 333, as the charge Qγ. As describedabove, when the external force along the positive direction of theγ-axis is applied to the surface of the third piezoelectric plate 331 orthe surface of the fourth piezoelectric plate 333, positive charge iscollected in the vicinity of the internal electrode 332. As a result,positive charge Qγ is output from the internal electrode 332. On theother hand, when the external force along the negative direction of theγ-axis is applied to the surface of the third piezoelectric plate 331 orthe surface of the fourth piezoelectric plate 333, negative charge iscollected in the vicinity of the internal electrode 332. As a result,negative charge Qγ is output from the internal electrode 332.

The first α-axis piezoelectric substance 340 has a function ofoutputting the charge Qα in accordance with the external force (shearingforce) parallel or substantially parallel to the α-axis. The firstα-axis piezoelectric substance 340 is configured to output positivecharge in accordance with the external force applied along the positivedirection of the α-axis, and to output negative charge in accordancewith the external force applied along the negative direction of theα-axis. That is, the first α-axis piezoelectric substance 340 has aelectric axis Pα facing the positive direction of the α-axis in FIGS.12A and 12B.

The first α-axis piezoelectric substance 340 includes a fifthpiezoelectric plate 341 having a fifth crystal axis CA5, a sixthpiezoelectric plate 343, provided facing the fifth piezoelectric plate341, which has a sixth crystal axis CA6, and an internal electrode 342,provided between the fifth piezoelectric plate 341 and the sixthpiezoelectric plate 343, which outputs the charge Qα. In addition, thelamination order of the respective layers constituting the first α-axispiezoelectric substance 340 is the order of the fifth piezoelectricplate 341, the internal electrode 342, and the sixth piezoelectric plate343 from the lower side in FIGS. 12A and 12B.

The fifth piezoelectric plate 341 is constituted by a piezoelectricsubstance having the fifth crystal axis CA5 oriented in the negativedirection of the α-axis. When the external force along the positivedirection of the α-axis is applied to the surface of the fifthpiezoelectric plate 341, charge is induced into the fifth piezoelectricplate 341 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the fifth piezoelectricplate 341 on the internal electrode 342 side, and negative charge iscollected in the vicinity of the surface of the fifth piezoelectricplate 341 on the ground electrode layer 310 side. Similarly, when theexternal force along the negative direction of the α-axis is applied tothe surface of the fifth piezoelectric plate 341, negative charge iscollected in the vicinity of the surface of the fifth piezoelectricplate 341 on the internal electrode 342 side, and positive charge iscollected in the vicinity of the surface of the fifth piezoelectricplate 341 on the ground electrode layer 310 side.

The sixth piezoelectric plate 343 is constituted by a piezoelectricsubstance having the sixth crystal axis CA6 oriented in the positivedirection of the α-axis. When the external force along the positivedirection of the α-axis is applied to the surface of the sixthpiezoelectric plate 343, charge is induced into the sixth piezoelectricplate 343 by a piezoelectric effect. As a result, positive charge iscollected in the vicinity of the surface of the sixth piezoelectricplate 343 on the internal electrode 342 side, and negative charge iscollected in the vicinity of the surface of the sixth piezoelectricplate 343 on the ground electrode layer 310 side. Similarly, when theexternal force along the negative direction of the α-axis is applied tothe surface of the sixth piezoelectric plate 343, negative charge iscollected in the vicinity of the surface of the sixth piezoelectricplate 343 on the internal electrode 342 side, and positive charge iscollected in the vicinity of the surface of the sixth piezoelectricplate 343 on the ground electrode layer 310 side.

As constituent materials of the fifth piezoelectric plate 341 and thesixth piezoelectric plate 343, the same constituent materials as thoseof the first piezoelectric plate 321 and the second piezoelectric plate323 can be used. In addition, the piezoelectric plate, such as the fifthpiezoelectric plate 341 and the sixth piezoelectric plate 343, whichgenerates charge by the external force (shearing force) applied alongthe surface direction of the layer can be formed of Y cut quartzcrystal, similarly to the first piezoelectric plate 321 and the secondpiezoelectric plate 323.

The internal electrode 342 has a function of outputting positive chargeor negative charge, generated within the fifth piezoelectric plate 341and the sixth piezoelectric plate 343, as the charge Qα. As describedabove, when the external force along the positive direction of theα-axis is applied to the surface of the fifth piezoelectric plate 341 orthe surface of the sixth piezoelectric plate 343, positive charge iscollected in the vicinity of the internal electrode 342. As a result,positive charge Qα is output from the internal electrode 342. On theother hand, when the external force along the negative direction of theα-axis is applied to the surface of the fifth piezoelectric plate 341 orthe surface of the sixth piezoelectric plate 343, negative charge iscollected in the vicinity of the internal electrode 342. As a result,negative charge Qα is output from the internal electrode 342.

The β-axis piezoelectric substance 320, the first γ-axis piezoelectricsubstance 330, and the first α-axis piezoelectric substance 340 arelaminated so that the direction of the electric axis Pβ of the β-axispiezoelectric substance 320, the direction of the electric axis Pγ ofthe first γ-axis piezoelectric substance 330, and the direction of theelectric axis Pα of the first α-axis piezoelectric substance 340 areorthogonal to each other. Thereby, the first charge output element 301 acan have three electric axes Pα, Pβ, and Pγ, and can output threecharges Qα, Qβ, and Qγ in accordance with the respective external forcesapplied along three axes (α-axis, β-axis, and γ-axis).

Next, the second charge output element 301 b included in each of theforce detection elements 30 b and 30 d belonging to the second elementpair will be described in detail with reference to FIG. 12B. The secondcharge output element 301 b shown in FIG. 12B has a function ofoutputting the charges Qα, Qβ, and Qγ in accordance with the respectiveexternal forces along the three axes (α-axis, β-axis, and γ-axis)orthogonal to each other in FIGS. 12A and 12B. As shown in FIG. 12B, thesecond charge output element 301 b includes four ground electrode layers310 grounded to a ground (reference potential point) GND, the β-axispiezoelectric substance 320 that outputs the charge Qβ in accordancewith the external force (shearing force) parallel or substantiallyparallel to the β-axis, a second γ-axis piezoelectric substance 350 thatoutputs the charge Qγ in accordance with the external force(compressive/tensile force) parallel or substantially parallel to theγ-axis, and a second α-axis piezoelectric substance 360 that outputs thecharge Qα in accordance with the external force (shearing force)parallel or substantially parallel to the α-axis, and the groundelectrode layers 310 and each of the piezoelectric substances 320, 350,and 360 are alternately laminated. Therefore, the second charge outputelement 301 b has the same structure as that of the first charge outputelement 301 a, except that the second charge output element includes thesecond γ-axis piezoelectric substance 350 having a structure differentfrom that of the first γ-axis piezoelectric substance 330 and the secondα-axis piezoelectric substance 360 having a structure different fromthat of the first α-axis piezoelectric substance 340. Meanwhile, inFIGS. 12A and 12B, the lamination direction of the ground electrodelayers 310 and each of the piezoelectric substances 320, 350, and 360 isset to a γ-axis direction, and the directions which are orthogonal tothe γ-axis direction and are orthogonal to each other are set to anα-axis direction and a β-axis direction, respectively.

In the shown configuration, the β-axis piezoelectric substance 320, thesecond γ-axis piezoelectric substance 350, and the second α-axispiezoelectric substance 360 are laminated in this order from the lowerside in FIGS. 12A and 12B, but the invention is not limited thereto. Thelamination order of the piezoelectric substances 320, 350, and 360 isarbitrary.

The second γ-axis piezoelectric substance 350 has a function ofoutputting the charge Qγ in accordance with the external force(compressive/tensile force) parallel or substantially parallel to theγ-axis. The second γ-axis piezoelectric substance 350 is configured tooutput negative charge in accordance with the external force appliedalong the positive direction of the γ-axis, and to output positivecharge in accordance with the external force applied along the negativedirection of the γ-axis. That is, the second γ-axis piezoelectricsubstance 350 has the electric axis Pγ facing the negative direction ofthe γ-axis in FIGS. 12A and 12B. Therefore, the direction of theelectric axis Pγ of the second γ-axis piezoelectric substance 350 isopposite to the direction of the electric axis Pγ of the first γ-axispiezoelectric substance 330.

The second γ-axis piezoelectric substance 350 includes the fourthpiezoelectric plate 333 having the fourth crystal axis CA4, the thirdpiezoelectric plate 331, provided facing the fourth piezoelectric plate333, which has the third crystal axis CA3, and the internal electrode332, provided between the fourth piezoelectric plate 333 and the thirdpiezoelectric plate 331, which outputs the charge Qγ. In addition, thelamination order of the respective layers constituting the second γ-axispiezoelectric substance 350 is the order of the fourth piezoelectricplate 333, the internal electrode 332, and the third piezoelectric plate331 from the lower side in FIGS. 12A and 12B. Therefore, the secondγ-axis piezoelectric substance 350 has the same structure as that of thefirst γ-axis piezoelectric substance 330, except for the laminationorder of the fourth piezoelectric plate 333, the internal electrode 332,and the third piezoelectric plate 331.

When the external force applied along the positive direction of theγ-axis is applied to the surface of the fourth piezoelectric plate 333,charge is induced into the fourth piezoelectric plate 333 by apiezoelectric effect. As a result, negative charge is collected in thevicinity of the surface of the fourth piezoelectric plate 333 on theinternal electrode 332 side, and positive charge is collected in thevicinity of the surface of the fourth piezoelectric plate 333 on theground electrode layer 310 side. Similarly, when the external forcealong the negative direction of the γ-axis is applied to the surface ofthe fourth piezoelectric plate 333, positive charge is collected in thevicinity of the surface of the fourth piezoelectric plate 333 on theinternal electrode 332 side, and negative charge is collected in thevicinity of the surface of the fourth piezoelectric plate 333 on theground electrode layer 310 side.

When the external force along the positive direction of the γ-axis isapplied to the surface of the third piezoelectric plate 331, charge isinduced into the third piezoelectric plate 331 by a piezoelectriceffect. As a result, negative charge is collected in the vicinity of thesurface of the third piezoelectric plate 331 on the internal electrode332 side, and positive charge is collected in the vicinity of thesurface of the third piezoelectric plate 331 on the ground electrodelayer 310 side. Similarly, when the external force along the negativedirection of the γ-axis is applied to the surface of the thirdpiezoelectric plate 331, positive charge is collected in the vicinity ofthe surface of the third piezoelectric plate 331 on the internalelectrode 332 side, and negative charge is collected in the vicinity ofthe surface of the third piezoelectric plate 331 on the ground electrodelayer 310 side.

In this manner, when the external force along the positive direction ofthe γ-axis is applied to the surface of the third piezoelectric plate331 or the surface of the fourth piezoelectric plate 333, negativecharge is collected in the vicinity of the internal electrode 332. As aresult, negative charge Qγ is output from the internal electrode 332. Onthe other hand, when the external force along the negative direction ofthe γ-axis is applied to the surface of the third piezoelectric plate331 or the surface of the fourth piezoelectric plate 333, positivecharge is collected in the vicinity of the internal electrode 332. As aresult, positive charge Qγ is output from the internal electrode 332.

The second α-axis piezoelectric substance 360 has a function ofoutputting the charge Qα in accordance with the external force (shearingforce) parallel or substantially parallel to the α-axis. The secondα-axis piezoelectric substance 360 is configured to output negativecharge in accordance with the external force along the positivedirection of the α-axis, and to output positive charge in accordancewith the external force applied along the negative direction of theα-axis. That is, the second α-axis piezoelectric substance 360 has theelectric axis Pα facing the negative direction of the α-axis in FIGS.12A and 12B. Therefore, the direction of the electric axis Pα of thesecond α-axis piezoelectric substance 360 is opposite to the directionof the electric axis Pα of the first α-axis piezoelectric substance 340.

The second α-axis piezoelectric substance 360 includes the sixthpiezoelectric plate 343 having the sixth crystal axis CA6, the fifthpiezoelectric plate 341, provided facing the sixth piezoelectric plate343, which has the fifth crystal axis CA5, and the internal electrode342, provided between the sixth piezoelectric plate 343 and the fifthpiezoelectric plate 341, which outputs the charge Qα. In addition, thelamination order of the respective layers constituting the second α-axispiezoelectric substance 360 is the order of the sixth piezoelectricplate 343, the internal electrode 342, and the fifth piezoelectric plate341 from the lower side in FIGS. 12A and 12B. Therefore, the secondα-axis piezoelectric substance 360 has the same structure as that of thefirst α-axis piezoelectric substance 340, except for the laminationorder of the sixth piezoelectric plate 343, the internal electrode 342,and the fifth piezoelectric plate 341.

When the external force along the positive direction of the α-axis isapplied to the surface of the sixth piezoelectric plate 343, charge isinduced into the sixth piezoelectric plate 343 by a piezoelectriceffect. As a result, negative charge is collected in the vicinity of thesurface of the sixth piezoelectric plate 343 on the internal electrode342 side, and positive charge is collected in the vicinity of thesurface of the sixth piezoelectric plate 343 on the ground electrodelayer 310 side. Similarly, when the external force along the negativedirection of the α-axis is applied to the surface of the sixthpiezoelectric plate 343, positive charge is collected in the vicinity ofthe surface of the sixth piezoelectric plate 343 on the internalelectrode 342 side, and negative charge is collected in the vicinity ofthe surface of the sixth piezoelectric plate 343 on the ground electrodelayer 310 side.

When the external force along the positive direction of the α-axis isapplied to the surface of the fifth piezoelectric plate 341, charge isinduced into the fifth piezoelectric plate 341 by a piezoelectriceffect. As a result, negative charge is collected in the vicinity of thesurface of the fifth piezoelectric plate 341 on the internal electrode342 side, and positive charge is collected in the vicinity of thesurface of the fifth piezoelectric plate 341 on the ground electrodelayer 310 side. Similarly, when the external force along the negativedirection of the α-axis is applied to the surface of the fifthpiezoelectric plate 341, positive charge is collected in the vicinity ofthe surface of the fifth piezoelectric plate 341 on the internalelectrode 342 side, and negative charge is collected in the vicinity ofthe surface of the fifth piezoelectric plate 341 on the ground electrodelayer 310 side.

In this manner, when the external force along the positive direction ofthe α-axis is applied to the surface of the fifth piezoelectric plate341 or the surface of the sixth piezoelectric plate 343, negative chargeis collected in the vicinity of the internal electrode 342. As a result,negative charge Qα is output from the internal electrode 342. On theother hand, when the external force along the negative direction of theα-axis is applied to the surface of the fifth piezoelectric plate 341 orthe surface of the sixth piezoelectric plate 343, positive charge iscollected in the vicinity of the internal electrode 342. As a result,positive charge Qα is output from the internal electrode 322.

The β-axis piezoelectric substance 320, the second γ-axis piezoelectricsubstance 350, and the second α-axis piezoelectric substance 360 arelaminated so that the direction of the electric axis Pβ of the β-axispiezoelectric substance 320, the direction of the electric axis Pγ ofthe second γ-axis piezoelectric substance 350, and the direction of theelectric axis Pα of the second α-axis piezoelectric substance 360 areorthogonal to each other. In addition, the direction of the electricaxis Pγ of the second γ-axis piezoelectric substance 350 is opposite tothe direction of the electric axis Pγ of the first γ-axis piezoelectricsubstance 330. Similarly, the direction of the electric axis Pα of thesecond α-axis piezoelectric substance 360 is opposite to the directionof the electric axis Pα of the first α-axis piezoelectric substance 340.

In addition, the amount of charge generation per unit force of theβ-axis piezoelectric substance 320, the first α-axis piezoelectricsubstance 340 and the second α-axis piezoelectric substance 360 whichare formed of Y cut quartz crystal is, for example, 8 pC/N. On the otherhand, the amount of charge generation per unit force of the first γ-axispiezoelectric substance 330 and the second γ-axis piezoelectricsubstance 350 which are formed of X cut quartz crystal is, for example,4 pC/N. In this manner, the sensitivity of the first charge outputelement 301 a and the second charge output element 301 b to the externalforce (compressive/tensile force) parallel or substantially parallel tothe γ-axis becomes equal to or less than the sensitivity of the firstcharge output element 301 a and the second charge output element 301 bto the external force (shearing force) parallel or substantiallyparallel to the α-axis or the β-axis. For this reason, the charge Qγwhich is output from the first charge output element 301 a and thesecond charge output element 301 b becomes equal to or less than thecharge Qα and the charge Qβ which are output from the first chargeoutput element 301 a and the second charge output element 301 b.

Conversion and Output Circuit

The conversion and output circuits 132 a and 132 c have the sameconfiguration as that of the conversion and output circuit 132 of thefourth embodiment. The conversion and output circuit 132 b has the sameconfiguration as that of the conversion and output circuit 132 of thefourth embodiment, except for the capacitance of the capacitor 134. Theconversion and output circuit 132 a has a function of converting thecharge Qα which is output from the first charge output element 301 a orthe second charge output element 301 b into the voltage Vα. Theconversion and output circuit 132 b has a function of converting thecharge Qγ which is output from the first charge output element 301 a orthe second charge output element 301 b into the voltage Vγ. Theconversion and output circuit 132 c has a function of converting thecharge Qβ which is output from the first charge output element 301 a orthe second charge output element 301 b into the voltage Vβ.

The same drive circuit may be connected to the switching elements 135 ofthe respective conversion and output circuits 132 a, 132 b, and 132 c,and different drive circuits may be connected thereto. All synchronizedon/off signals are input to the respective switching elements 135 fromthe drive circuit. Thereby, the operations of the switching elements 135of the respective conversion and output circuits 132 a, 132 b, and 132 care synchronized with each other. That is, the on/off timings of theswitching elements 135 of the respective conversion and output circuits132 a, 132 b, and 132 c are consistent with each other.

In addition, when the capacitance of the capacitor 134 is reduced in thecircuit, such as the conversion and output circuits 132 a, 132 b, and132 c, which has a voltage conversion function, voltage conversionsensitivity is improved, but the amount of saturated charge is reduced.As described above, generally, the charge Qγ which is output from thefirst charge output element 301 a and the second charge output element301 b is equal to or less than the charge Qα and the charge Qβ which areoutput from the first charge output element 301 a and the second chargeoutput element 301 b. Therefore, from the viewpoint of sensitivity tothe charge Qγ, it is preferable that the capacitance C2 of the capacitor134 of the conversion and output circuit 132 b is equal to or less thanthe capacitance C1 of the capacitor 134 of the conversion and outputcircuits 132 a and 132 c. Thereby, it is possible to accurately convertthe charge Qγ into the voltage Vγ.

In addition, the switching elements 135 of the respective conversion andoutput circuits 132 a, 132 b, and 132 c are the same semiconductorswitching elements as each other, the leakage currents of the respectiveswitching elements 135 are substantially equal to each other. Therefore,the output drifts D of the respective switching elements 135 are alsosubstantially equal to each other.

Next, the positional relations between the force detection elements 30 aand 30 c constituting the first element pair and the force detectionelements 30 b and 30 d constituting the second element pair will bedescribed in detail with reference to FIG. 10B. Meanwhile, in FIG. 10B,the cover plate 4 is omitted for the purpose of description. Inaddition, in FIG. 10B, a horizontal direction is set to an x-axisdirection, a direction orthogonal to the x-axis direction, that is, avertical direction is set to a y-axis direction, and a directionorthogonal to the x-axis direction and the y-axis direction is set to az-axis direction.

The force detection element 30 a has electric axes Pα1, Pβ1, and Pγ1,and outputs voltages Vα1, Vβ1, and Vγ1 in accordance with externalforces applied along the α-axis, the β-axis, and the γ-axis,respectively. The force detection element 30 b has electric axes Pα2,Pβ2, and Pγ2, and outputs voltages Vα2, Vβ2, and Vγ2 in accordance withthe external forces applied along the α-axis, the β-axis, and theγ-axis, respectively. The force detection element 30 c has electric axesPα3, Pβ3, and Pγ3, and outputs voltages Vα3, Vβ3, and Vγ3 in accordancewith the external forces applied along the α-axis, the β-axis, and theγ-axis, respectively. Similarly, the force detection element 30 d haselectric axes Pα4, Pβ4, and Pγ4, and outputs voltages Vα4, Vβ4, and Vγ4in accordance with the external forces applied along the α-axis, theβ-axis, and the γ-axis, respectively. In addition, voltage components(true values) Vαt, Vβt, and Vγt proportional to the amount of chargeaccumulated in the capacitor 134, and the output drift D caused by theleakage current of the switching element 135 are respectively includedin the voltages Vα, Vβ, and Vγ which are output by the force detectionelements 30 a, 30 b, 30 c, and 30 d.

The force detection elements 30 a, 30 b, 30 c, and 30 d are provided(interposed) between the base plate 2 and the cover plate 4 providedseparately from the base plate 2. The electric axis Pβ1 of the forcedetection element 30 a has an angle θ1. The electric axis Pβ2 of theforce detection element 30 b has an angle θ2. The electric axis Pβ3 ofthe force detection element 30 c has an angle θ3. The electric axis Pβ4of the force detection element 30 d has an angle θ4. Meanwhile, theangles θ1, θ2, θ3, and θ4 are angles from the x-axis of the referencecoordinate system (x-axis, y-axis, and z-axis) of FIG. 10B.

As shown in FIG. 10B, the force detection elements 30 a and 30 cconstituting the first element pair are disposed so that the directionsof the electric axes Pα1 and Pβ1 of the force detection element 30 a andthe directions of the electric axes Pα3 and Pβ3 of the force detectionelement 30 c are opposite to each other. Similarly, the force detectionelements 30 b and 30 d constituting the second element pair are disposedso that the directions of the electric axes Pα2 and Pβ2 of the forcedetection element 30 b and the directions of the electric axes Pα4 andPβ4 of the force detection element 30 d are opposite to each other. Inaddition, the directions of the electric axes Pγ1 and Pγ3 of the forcedetection elements 30 a and 30 c constituting the first element pair andthe directions of the electric axes Pγ2 and Pγ4 of the force detectionelements 30 b and 30 d constituting the second element pair are oppositeto each other.

Since the force detection elements 30 a and 30 c are disposed so thatthe direction of the electric axis Pβ1 of the force detection element 30a and the direction of the electric axis Pβ2 of the force detectionelement 30 c are opposite to each other, the sign of a voltage componentVβt1 included in the voltage Vβ1 and the sign of a voltage componentVβt3 included in the voltage Vβ3 are not consistent with each other.Therefore, when the difference between the voltage Vβ1 which is outputfrom the force detection element 30 a and the voltage Vβ3 which isoutput from the force detection element 30 c is taken, the absolutevalue of the difference between the voltage component Vβt1 and thevoltage component Vβt3 does not decrease. Similarly, since the forcedetection elements 30 a and 30 c are disposed so that the direction ofthe electric axis Pα1 of the force detection element 30 a and thedirection of the electric axis Pα2 of the force detection element 30 care opposite to each other, the sign of a voltage component Vαt1included in the voltage Vα1 and the sign of a voltage component Vαt3included in the voltage Vα3 are not consistent with each other.Therefore, when the difference between the voltage Vα1 which is outputfrom the force detection element 30 a and the voltage Vα3 which isoutput from the force detection element 30 c is taken, the absolutevalue of the difference between the voltage component Vαt1 and thevoltage component Vαt3 does not decrease.

In addition, since the force detection elements 30 b and 30 d aredisposed so that the direction of the electric axis Pβ2 of the forcedetection element 30 b and the direction of the electric axis Pβ4 of theforce detection element 30 d are opposite to each other, the sign of avoltage component Vβt2 included in the voltage 132 and the sign of avoltage component Vβt4 included in the voltage 134 are not consistentwith each other. Therefore, when the difference between the voltage Vβ2which is output from the force detection element 30 b and the voltageVβ4 which is output from the force detection element 30 d is taken, theabsolute value of the difference between the voltage component Vβt2 andthe voltage component Vβt4 does not decrease. Similarly, since the forcedetection elements 30 b and 30 d are disposed so that the direction ofthe electric axis Pα2 of the force detection element 30 b and thedirection of the electric axis Pα4 of the force detection element 30 dare opposite to each other, the sign of a voltage component Vαt2included in the voltage Vα2 and the sign of a voltage component Vαt4included in the voltage Vα4 are not consistent with each other.Therefore, when the difference between the voltage Vα2 which is outputfrom the force detection element 30 b and the voltage Vα4 which isoutput from the force detection element 30 d is taken, the absolutevalue of the difference between the voltage component Vαt2 and thevoltage component Vαt4 does not decrease.

In addition, as described above, since the force detection elements 30a, 30 c, 30 b, and 30 d are configured such that the directions of theelectric axes Pγ1 and Pγ3 of the force detection elements 30 a and 30 cand the direction of the electric axes Pγ2 and Pγ4 of the forcedetection elements 30 b and 30 d are opposite to each other, the sign ofvoltage components Vγt1 and Vγt3 included in the voltages Vγ1 and Vγ3and the sign of voltage components Vγt2 and Vγt4 included in thevoltages Vγ2 and Vγ4 are not consistent with each other. Therefore, whenthe differences between the voltages Vγ1 and Vγ3 which are output fromthe force detection elements 30 a and 30 c and the voltages Vγ2 and Vγ4which are output from the force detection elements 30 b and 30 d aretaken, the absolute values of these differences do not decrease.

On the other hand, since the output drifts D included in the voltagesVα, Vβ, and Vγ which are output from the force detection elements 30 a,30 b, 30 c, and 30 d are independent of the directions of the electricaxes Pα, Pβ, and Pγ, the signs of the output drifts D included in thevoltages Vα, Vβ, and Vγ are consistent with each other. Therefore, whenthe difference between the output drifts D is taken, the absolute valueof the difference decreases.

In addition, it is preferable that the force detection elements 30 a and30 c constituting the first element pair are disposed so that thedirection of the electric axis Pβ1 and the direction of the electricaxis Pβ3 face each other, that is, the relation of θ1=θ3 is satisfied.Similarly, it is preferable that the force detection elements 30 b and30 d constituting the second element pair are disposed so that thedirection of the electric axis Pβ2 and the direction of the electricaxis Pβ4 face each other, that is, the relation of θ2=θ4 is satisfied.Thereby, an external force detection circuit 150 described later candetect six-axis forces while reducing the output drift D.

In addition, it is more preferable that the force detection elements 30a, 30 b, 30 c, and 30 d are disposed so that the directions of theelectric axes Pβ1 and Pβ3 of the force detection elements 30 a and 30 cconstituting the first element pair and the directions of the electricaxes Pβ2 and Pβ4 of the force detection elements 30 b and 30 dconstituting the second element pair are orthogonal to each other.Thereby, the external force detection circuit 150 described later candetect six-axis forces while further reducing the output drift D.

In addition, when the force detection elements are disposed so that thedirection of the electric axis Pβ1 of the force detection element 30 aand the direction of the electric axis Pβ3 of the force detectionelement 30 c are opposite to each other, the arrangement of the forcedetection elements 30 a and 30 c constituting the first element pair isnot particularly limited, but it is preferable that the force detectionelement 30 a and the force detection element 30 c are disposed on thesame axis A1 as shown in FIG. 10B. Similarly, when the force detectionelements are disposed so that the direction of the electric axis Pβ2 ofthe force detection element 30 b and the direction of the electric axisPβ4 of the force detection element 30 d are opposite to each other, thearrangement of the force detection elements 30 b and 30 d constitutingthe second element pair is not particularly limited, but it ispreferable that the force detection element 30 b and the force detectionelement 30 d are disposed on the same axis A2 as shown in FIG. 10B.Thereby, the six-axis forces applied to the base plate 2 or the coverplate 4 can be detected in an unbiased manner.

In addition, the positional relation between the first element pair andthe second element pair is not particularly limited. However, as shownin FIG. 10B, it is preferable that the first element pair and the secondelement pair are disposed so that the axis A1 connecting the center ofthe force detection element 30 a belonging to the first element pair tothe center of the force detection element 30 c belonging thereto and theaxis A2 connecting the center of the force detection element 30 bbelonging to the second element pair to the center of the forcedetection element 30 d belonging thereto are orthogonal to each other.Thereby, the external forces (external forces applied along the x-axis,the y-axis, and the z-axis in the drawing) applied to the base plate 2or the cover plate 4 can be detected in an unbiased manner.

In addition, the force detection elements 30 a, 30 b, 30 c, and 30 d arepreferably disposed at equal angular intervals along the circumferentialdirection of the base plate 2 or the cover plate 4, and are morepreferably disposed at equal intervals concentrically about the centralpoint of the base plate 2 or the cover plate 4. Thereby, the externalforces (external forces applied along the x-axis, the y-axis, and thez-axis in the drawing) applied to the base plate 2 or the cover plate 4can be detected in an unbiased manner.

In addition, in the configuration of FIG. 10B, the electric axes Pβ1,Pβ2, Pβ3, and Pβ4 of the force detection elements 30 a, 30 b, 30 c, and30 d face the outside (centrifugal direction) of the base plate 2, butthe invention is not limited thereto. That is, when the force detectionelements are disposed so that the direction of the electric axis Pβ1 ofthe force detection element 30 a and the direction of the electric axisPβ3 of the force detection element 30 c are opposite to each other, andthe direction of the electric axis Pβ2 of the force detection element 30b and the direction of the electric axis Pβ4 of the force detectionelement 30 d are opposite to each other, the electric axes Pβ1, Pβ2,Pβ3, and Pβ4 of the force detection elements 30 a, 30 b, 30 c, and 30 dmay face the central direction (centripetal direction) of the base plate2. Thereby, the electric axes Pα1, Pα2, Pα3, and Pα4 of the forcedetection elements 30 a, 30 b, 30 c, and 30 d face the connectiondirection of a circle centered around the central point of the baseplate 2. Therefore, the external force detection circuit 50 describedlater can easily detect a rotational force component Mz about thez-axis.

External Force Detection Circuit

The external force detection circuit 50 has a function of arithmeticallyoperating six-axis forces of a translational force component (shearingforce) Fx in the x-axis direction, a translational force component(shearing force) Fy in the y-axis direction, a translational forcecomponent (compressive/tensile force) Fz in the z-axis direction, arotational force component Mx about the x-axis, a rotational forcecomponent My about the y-axis, and a rotational force component Mz aboutthe z-axis by taking the differences between the voltages Vα, Vβ, and Vγwhich are output from the force detection elements 30 a, 30 b, 30 c, and30 d. The respective force components can be obtained by the followingexpressions. Meanwhile, in order to simplify the expressions, as shownin FIG. 10B, the force detection elements 30 a, 30 b, 30 c, and 30 d aredisposed concentrically with a radius L centered around the centralpoint of the base plate 2 or the cover plate 4, but the invention is notlimited thereto.

F_(x) = {V β 3 cos (θ3) − V β 1 cos (θ1)} + {V β 2 cos (θ2) − V β 4 cos (θ4)} + {V α1 cos (π/2 − θ1) − V α 3 cos (π/2 − θ3)} + {V α4 cos (π/2 − θ4) − V α 2 cos (π/2 − θ2)} = {V β t 3 cos (θ3) − V β t 1 cos (θ1)} + {V β t 2 cos (θ2) − V β t 4 cos (θ4)} + {V α t 1 cos (π/2 − θ1) − V α t 3 cos (π/2 − θ3)} + {V α t 4 cos (π/2 − θ4) − V α t 2 cos (π/2 − θ2)} + D{cos (θ3) − cos (θ1) + cos (θ2) − cos (θ4)} + D{cos (π/2 − θ1) − cos (π/2 − θ3) + cos (π/2 − θ4) − cos (π/2 − θ2)}F_(y) = {V β 1sin (θ1) − V β 3 sin (θ3)} + {V β 2 sin (θ2) − V β 4 sin (θ4)} + {V α1 sin (π/2 − θ1) − V α 3 sin (π/2 − θ3)} + {V α2 sin (π/2 − θ2) − V α 4 sin (π/2 − θ4)} = {V β t 1 sin (θ1) − V β t 3 sin (θ3)} + {V β t 2 sin (θ2) − V β t4 sin (θ4)} + {V α t 1 sin (π/2 − θ1) − V α t 3 sin (π/2 − θ3)} + {V α t 2 sin (π/2 − θ2) − V α t 4 sin (π/2 − θ4)} + D{sin (θ1) − sin (θ3) + sin (θ2) − sin (θ4)} + D{sin (π/2 − θ1) − sin (π/2 − θ3) + sin (π/2 − θ2) − sin (π/2 − θ4)}F_(s) = V γ 1 − V γ 2 + V γ 3 − V γ 4 = (V γ t 1 + D) − (V γ t 2 + D) + (V γ t 3 + D) − (V γ t 4 + D) = V γ t 1 − V γ t 2 + V γ t 3 − V γ t 4M_(x) = L × {−V γ 1cos (θ1 + 3π/2) + V γ 2cos (θ2 + π/2) + V γ 3cos (θ3 + 3π/2) − V γ4cos (θ4 + π/2)} = L × {−V γ t 1cos (θ1 + 3π/2) + V γ t 2cos (θ2 + π/2) + V γ t 3cos (θ3 + 3π/2) − V γ t 4cos (θ4 + π/2)} + L × D{−cos (θ1 + 3π/2) + cos (θ2 + π/2) + cos (θ3 + 3π/2) − cos (θ4 + π/2)}M_(y) = L × {V γ 1sin (θ1 + π/2) + V γ 2sin (θ2 + π/2) − V γ 3sin (θ3 + π/2) − V γ 4sin (θ4 + π/2)} = L × {V γ t 1sin (θ1 + π/2) + V γ t 2sin (θ2 + π/2) − V γ t 3sin (θ3 + π/2) − V γt4sin(θ4 + π/2)} + L × D{sin (θ1 + π/2) + sin (θ2 + π/2) − sin (θ3 + π/2) − sin (θ4 + π/2)}M_(s) = L × {V α 1 − V α 2 + V α 3 − V α 4} = L × {(V α t 1 + D) − (V α t 2 + D) + (V α t 3 + D) − (V α t 4 + D)} = L × (V α t 1 − V α t 2 + V α t 3 − V α t 4)

Herein, L is a constant.

In this manner, the differences between the voltages Vα, Vβ, and Vγwhich are output from the force detection elements 30 a, 30 b, 30 c, and30 d are taken, and thus the absolute value of the differences betweenthe voltage components (true values) Vαt, Vβt, and Vγt proportional tothe amount of charge accumulated in the capacitor 134 is not reduced,but the absolute value of the output drift D can be reduced. As aresult, it is possible to reduce the output drift D, and to improve thedetection accuracy and detection resolution of the force detectiondevice 101 b. In addition, a method of reducing the above-mentionedoutput drift D is effective even when the measurement time gets longer,and thus it is possible to lengthen the measurement time of the forcedetection device 101 b.

Further, when the angles θ1, θ2, θ3, and θ4 satisfy the relations ofθ1=θ3 and θ2=θ4, the above calculation expressions are simplified asfollows.

F_(x) = {V β t 3 cos (θ3) − V β t 1 cos (θ1)} + {V βt 2 cos (θ2) − V β t 4 cos (θ4)} + {V αt1 cos (π/2 − θ1) − V αt 3 cos (π/2 − θ3)} + {V αt4 cos (π/2 − θ4) − V αt 2 cos (π/2 − θ2)}F_(y) = {V β t 1 sin (θ1) − V β t 3 sin (θ3)} + {V βt 2 sin (θ2) − V β t 4 sin (θ4)} + {V αt1 sin (π/2 − θ1) − V αt 3 sin (π/2 − θ3)} + {V αt2 sin (π/2 − θ2) − V αt4sin(π/2 − θ4)}     F_(s) = V γ t 1 − V γ t 2 + V γ t 3 − V γ t 4M_(x) = L × {−V γ t 3 cos (θ1 + 3π/2) + V γ t 2cos (θ2 + 3π/2) + V γ t 3cos (θ3 + 3π/2) − V γ t 4cos (θ4 + 3π/2)}M_(y) = L × {V γ t 1sin (θ1 + π/2) + V γ t 2sin (θ2 + π/2) − V γ t 3sin (θ3 + π/2) − V β t4sin (θ4 + π/2)}  M_(s) = L × (V α t 1 − V α t 2 + V α t 3 − V α t 4)

In this case, it is possible to remove the output drift D. As a result,it is possible to further improve the detection accuracy and detectionresolution of the force detection device 101 b. In addition, it ispossible to further lengthen the measurement time of the force detectiondevice 101 b.

Further, when the angles θ1, θ2, θ3, and θ4 satisfy the relations ofθ1=θ3=n/2 and θ2=θ4=0, the above calculation expressions are furthersimplified as follows.

F _(x) =Vβt2−Vβt4+Vαt1−Vαt3

F _(y) =Vβt1−Vβt3+Vαt2−Vαt4

F _(z) =Vγt1−Vγt2+Vγt3−Vγt4

M _(x) =L×(−Vγt1+Vγt3)

M _(y) =L×(Vγt2−Vγt4)

M _(z) =L×(Vαt1−Vαt2+Vαt3−Vαt4)

In this manner, the external force detection circuit 150 takes thedifferences between the voltages Vα, Vβ, and Vγ which are output fromthe force detection elements 30 a, 30 b, 30 c, and 30 d, and thus candetect six-axis forces while reducing the output drift D caused by theleakage current of the switching element 135 of each of the conversionand output circuits 132 a, 32 b, and 32 c. As a result, a detectionerror caused by the leakage current (output drift D) becomes relativelysmall, and thus it is possible to improve the detection accuracy anddetection resolution of the force detection device 101 b. In addition,the method of reducing the above-mentioned output drift D is effectiveeven when the measurement time gets longer, and thus it is possible tolengthen the measurement time of the force detection device 101 b.Further, in the force detection device 101 b of the embodiment, since acircuit, such as a reverse bias circuit, for reducing the output driftis not required, it is possible to reduce the size of the forcedetection device 101 b.

Meanwhile, the force detection device 101 b of the embodiment includestwo element pairs of the force detection elements 30 a and 30 cconstituting the first element pair and the force detection elements 30c and 30 d constituting the second element pair, but the invention isnot limited thereto. When the force detection device 101 b includes twoelement pairs of the first element pair and the second element pair asshown in FIG. 10B, six-axis forces can be obtained by a very simplearithmetic operation as described above, and thus it is possible tosimplify the external force detection circuit 50. In addition, when theforce detection device 101 b includes three or more element pairs, it ispossible to detect the six-axis forces with a higher degree of accuracy.

Sixth Embodiment

Next, a single-arm robot which is a sixth embodiment of the inventionwill be described with reference to FIG. 13. Hereinafter, the sixthembodiment will be described with an emphasis on the difference with theabove-mentioned embodiment, and the description of the same particularswill be omitted.

FIG. 13 is a diagram illustrating an example of the single-arm robotusing a force detection device 1 (1 a, 1 b, 101 a or 101 b) according tothe invention. A single-arm robot 500 of FIG. 13 includes a base 510, anarm connecting body 520, an end effector 530 provided at the tip side ofthe arm connecting body 520, and the force detection device 1 (1 a, 1 b,101 a or 101 b) provided between the arm connecting body 520 and the endeffector 530.

The base 510 has a function of receiving an actuator (not shown) thatgenerates power for rotating the arm connecting body 520, a controlportion (not shown) that controls the actuator, and the like. Inaddition, the base 510 is fixed onto, for example, a floor, a wall, aceiling, a movable carriage, and the like.

The arm connecting body 520 includes a first arm 521, a second arm 522,a third arm 523, a fourth arm 524 and a fifth arm 525, and is configuredby rotatably connecting the adjacent arms. The arm connecting body 520is driven through complex rotation or flexion about the connectionportion of each arm by the control of the control portion.

The end effector 530 has a function of grasping an object. The endeffector 530 includes a first finger 531 and a second finger 532. Theend effector 530 reaches a predetermined operation position through thedriving of the arm connecting body 520, and then the separation distancebetween the first finger 531 and the second finger 532 is adjusted,thereby allowing an object to be grasped.

The force detection device 1 uses any of the force detection devices 1a, 1 b, 101 a, and 101 b of the above-mentioned embodiment, and has afunction of detecting an external force applied to the end effector 530.The external force detected by the force detection device 1 is fed backto the control portion of the base 510, and thus the single-arm robot500 can execute more precise work. In addition, the single-arm robot 500can detect the contact of the end effector 530 to an obstacle, and thelike, through six-axis forces detected by the force detection device 1.Therefore, it is possible to easily perform an obstacle avoidanceoperation, an object damage avoidance operation, and the like which aredifficult to perform in the position control of the related art, and thesingle-arm robot 500 can execute work more safely. Further, in the forcedetection device 1 of the embodiment, since a circuit, such as a reversebias circuit, for reducing the output drift is not required, it ispossible to reduce the size of the force detection device 1. Therefore,it is possible to reduce the size of the single-arm robot 500.

Meanwhile, in the shown configuration, the arm connecting body 520 isconstituted by a total of five arms, but the invention is not limitedthereto. Cases where the arm connecting body 520 is constituted by onearm, is constituted by two to four arms, and is constituted by six ormore arms are also within the scope of the invention.

Seventh Embodiment

Next, a moving object which is a seventh embodiment of the inventionwill be described with reference to FIG. 14. Hereinafter, the seventhembodiment will be described with an emphasis on the difference with theabove-mentioned embodiment, and the description of the same particularswill be omitted.

FIG. 14 is a diagram illustrating an example of the moving object usingthe aforementioned force detection device 1 (1 a, 1 b, 101 a or 101 b).A moving object 900 of FIG. 14 can move through given power. The movingobject 900, though not particularly limited, includes, for example,vehicles such as an automobile, a motorcycle, an airplane, a ship, and atrain, robots such as a bipedal walking robot and a wheel moving robot,and the like.

The moving object 900 includes a main body 910 (such as, for example, ahousing of a vehicle and a main body of a robot), a power output portion920 that supplies power for moving the main body 910, the forcedetection device 1 (1 a, 1 b, 101 a or 101 b) that detects an externalforce which is generated by the movement of the main body 910, and acontrol portion 930.

When the main body 910 moves through the power which is supplied fromthe power output portion 920, vibration, acceleration and the like aregenerated with the movement. The force detection device 1 detects anexternal force caused by vibration, acceleration and the like which aregenerated with the movement. The external force detected by the forcedetection device 1 is transmitted to the control portion 930. Thecontrol portion 930 controls the power output portion 920 and the likein accordance with the external force transmitted from the forcedetection device 1, and thus can execute control such as posturecontrol, vibration control and acceleration control. Further, in theforce detection device 1, since a circuit, such as a reverse biascircuit, for reducing the output drift is not required, it is possibleto reduce the size of the force detection device 1. Therefore, it ispossible to reduce the size of the moving object 900.

In addition, the force detection device 1 (1 a, 1 b, 101 a, and 101 b)can also be applied to various types of measurement instruments such asa vibrometer, an accelerometer, a gravimeter, a dynamometer, aseismometer or a clinometer, and various types of measurementinstruments using the force detection device 1 are also within the scopeof the invention.

As stated above, the force detection device of the invention, and therobot and the moving object using the force detection device have beendescribed on the basis of the shown embodiments, but the invention isnot limited thereto, and the configuration of each portion can bereplaced by any configuration having the same function. In addition, anyother configurations may be added to the invention. In addition, theinvention may be configured such that any two or more configurations(features) in the above embodiments are combined.

What is claimed is:
 1. A force detection device comprising: a chargeoutput element that outputs charge in accordance with an external force;a conversion and output circuit, having a first capacitor, whichconverts the charge into a voltage and outputs the voltage; acompensation signal output circuit, having a second capacitor, whichoutputs a compensation signal; and an external force detection circuitthat detects the external force on the basis of the voltage which isoutput from the conversion and output circuit and the compensationsignal which is output from the compensation signal output circuit,wherein a capacitance of the second capacitor is smaller than acapacitance of the first capacitor.
 2. The force detection deviceaccording to claim 1, wherein when the capacitance of the firstcapacitor is set to C1, and the capacitance of the second capacitor isset to C2, C2/C1 is 0.1 to 0.8.
 3. The force detection device accordingto claim 1, wherein the external force detection circuit includes a gaincorrection portion that gives a gain to at least one of the voltagewhich is output from the conversion and output circuit and thecompensation signal which is output from the compensation signal outputcircuit, to perform correction, and the external force detection circuitdetects the external force on the basis of the voltage corrected by thegain correction portion and the compensation signal.
 4. A robotcomprising: at least one arm connecting body having a plurality of armsand configured to rotatable connect adjacent arms of the plurality ofarms; an end effector which is provided at a tip side of the armconnecting body; and the force detection device according to claim 1,which is provided between the arm connecting body and the end effector,and detects an external force applied to the end effector.
 5. A robotcomprising: at least one arm connecting body having a plurality of armsand configured to rotatable connect adjacent arms of the plurality ofarms; an end effector which is provided at a tip side of the armconnecting body; and the force detection device according to claim 2,which is provided between the arm connecting body and the end effector,and detects an external force applied to the end effector.